os/ObjectStore: add get_devices() method

The intent here is to return the set of hardware storage devices, as
identified by the "devname" (i.e., the directory name for the device in
/sys/block/).  For example, "sda", "nvme0n1", etc.

Signed-off-by: Sage Weil <sage@redhat.com>

src/os/ObjectStore.h

@@ -1562,6 +1562,11 @@ public:
   virtual bool wants_journal() = 0;  //< prefers a journal
   virtual bool allows_journal() = 0; //< allows a journal
 
+  /// enumerate hardware devices (by 'devname', e.g., 'sda' as in /sys/block/sda)
+  virtual int get_devices(std::set<string> *devls) {
+    return -EOPNOTSUPP;
+  }
+
   /**
    * is_rotational
    *

src/os/memstore/MemStore.h

@@ -280,6 +280,11 @@ public:
     return false;
   }
 
+  int get_devices(set<string> *ls) override {
+    // no devices for us!
+    return 0;
+  }
+
   int statfs(struct store_statfs_t *buf) override;
 
   bool exists(const coll_t& cid, const ghobject_t& oid) override;