scripts/qemustart: allow machine selection with new option --machine

This can be used to set machine options like highmem=off for running old
armvirt/32 kernel lacking LPAE support with QEMU version 3.0 or later

 [1] Armv7 guest fails to boot with qemu-3.0.0-1,
     https://bugzilla.redhat.com/show_bug.cgi?id=1633328#c5

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>

scripts/qemustart

@@ -88,6 +88,7 @@ Usage: $SELF [-h|--help]
          [<subtarget> [<extra-qemu-options>]]
          [--kernel <kernel>]
          [--rootfs <rootfs>]
+         [--machine <machine>]
 
 <subtarget> will default to "generic" and must be specified if
 <extra-qemu-options> are present
@@ -100,13 +101,14 @@ the actual <target> in use.  They will default to files under bin/targets/
 Examples
 
   $SELF x86 64
-  $SELF x86 64 -enable-kvm -device virtio-balloon-pci
+  $SELF x86 64 --machine q35,accel=kvm -device virtio-balloon-pci
   $SELF x86 64 -incoming tcp:0:4444
   $SELF x86 64-glibc
   $SELF malta be -m 64
   $SELF malta le64
   $SELF malta be-glibc
   $SELF armvirt 32 \\
+                --machine virt,highmem=off \\
                 --kernel bin/targets/armvirt/32/openwrt-armvirt-32-zImage \\
                 --rootfs bin/targets/armvirt/32/openwrt-armvirt-32-root.ext4
 EOF
@@ -122,6 +124,7 @@ parse_args() {
 		case "$1" in
 			--kernel) o_kernel="$2"; shift 2 ;;
 			--rootfs) o_rootfs="$2"; shift 2 ;;
+			--machine|-machine|-M) o_mach="$2"; shift 2 ;;
 			--help|-h)
 				usage
 				exit 0
@@ -152,6 +155,7 @@ parse_args() {
 start_qemu_armvirt() {
 	local kernel="$o_kernel"
 	local rootfs="$o_rootfs"
+	local mach="${o_mach:-virt}"
 	local cpu
 	local qemu_exe
 
@@ -181,7 +185,7 @@ start_qemu_armvirt() {
 		)
 	}
 
-	"$qemu_exe" -machine virt -cpu "$cpu" -nographic \
+	"$qemu_exe" -machine "$mach" -cpu "$cpu" -nographic \
 		-netdev bridge,id=lan,br="$BR_LAN,helper=$HELPER" -device virtio-net-pci,id=devlan,netdev=lan,mac="$MAC_LAN" \
 		-netdev bridge,id=wan,br="$BR_WAN,helper=$HELPER" -device virtio-net-pci,id=devwan,netdev=wan,mac="$MAC_WAN" \
 		-kernel "$kernel" \
@@ -193,6 +197,7 @@ start_qemu_malta() {
 	local isel
 	local qemu_exe
 	local kernel="$o_kernel"
+	local mach="${o_mach:-malta}"
 
 	# o_subtarget can be le, be, le64, be64, le-glibc, le64-glibc, etc..
 	is64="$(echo $o_subtarget | grep -o 64)"
@@ -204,7 +209,7 @@ start_qemu_malta() {
 	# NOTE: order of wan, lan -device arguments matters as it will affect which
 	# one will be actually used as the wan, lan network interface inside the
 	# guest machine
-	"$qemu_exe" -machine malta -nographic \
+	"$qemu_exe" -machine "$mach" -nographic \
 		-netdev bridge,id=wan,br="$BR_WAN,helper=$HELPER" -device pcnet,netdev=wan,mac="$MAC_WAN" \
 		-netdev bridge,id=lan,br="$BR_LAN,helper=$HELPER" -device pcnet,netdev=lan,mac="$MAC_LAN" \
 		-kernel "$kernel" \
@@ -213,6 +218,7 @@ start_qemu_malta() {
 
 start_qemu_x86() {
 	local rootfs="$o_rootfs"
+	local mach="${o_mach:-pc}"
 	local qemu_exe
 
 	[ -n "$rootfs" ] || {
@@ -227,7 +233,7 @@ start_qemu_x86() {
 	# 64: 64-bit, kvm guest, virtio
 	#
 	case "${o_subtarget%-*}" in
-		legacy)			qemu_exe="qemu-system-i386"		;;
+		legacy)			qemu_exe="qemu-system-i386"	;;
 		generic|64)		qemu_exe="qemu-system-x86_64"	;;
 		*)
 			__errmsg "target $o_target: unknown subtarget $o_subtarget"
@@ -247,7 +253,7 @@ start_qemu_x86() {
 			#	-drive "file=$rootfs,format=raw,id=drv0,if=none" \
 			#
 			# [1] https://dev.openwrt.org/ticket/17947
-			"$qemu_exe" -nographic \
+			"$qemu_exe" -machine "$mach" -nographic \
 				-netdev bridge,id=lan,br="$BR_LAN,helper=$HELPER" -device e1000,id=devlan,netdev=lan,mac="$MAC_LAN" \
 				-netdev bridge,id=wan,br="$BR_WAN,helper=$HELPER" -device e1000,id=devwan,netdev=wan,mac="$MAC_WAN" \
 				-device ide-drive,drive=drv0 \
@@ -255,7 +261,7 @@ start_qemu_x86() {
 				"${o_qemu_extra[@]}"
 			;;
 		generic|64)
-			"$qemu_exe" -nographic \
+			"$qemu_exe" -machine "$mach" -nographic \
 				-netdev bridge,id=lan,br="$BR_LAN,helper=$HELPER" -device virtio-net-pci,id=devlan,netdev=lan,mac="$MAC_LAN" \
 				-netdev bridge,id=wan,br="$BR_WAN,helper=$HELPER" -device virtio-net-pci,id=devwan,netdev=wan,mac="$MAC_WAN" \
 				-drive "file=$rootfs,format=raw,if=virtio" \