virtio-devices: Add support for adding a single memory region

Assuming vhost-user devices support CONFIGURE_MEM_SLOTS protocol
feature, we introduce a new method to the VirtioDevice trait in order to
update one single memory at a time.

In case CONFIGURE_MEM_SLOTS is not supported by the backend (feature not
acked), we fallback onto the current way of updating the memory
mappings, that is with SET_MEM_TABLE.

Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>

virtio-devices/src/lib.rs

@@ -119,6 +119,7 @@ pub enum Error {
     EpollWait(io::Error),
     FailedSignalingDriver(io::Error),
     VhostUserUpdateMemory(vhost_user::Error),
+    VhostUserAddMemoryRegion(vhost_user::Error),
     EventfdError(io::Error),
     SetShmRegionsNotSupported,
     EpollHander(String),