virtio: reset region cache when on queue deletion

virtio: reset region cache when on queue deletion

https://bugzilla.redhat.com/show_bug.cgi?id=1708480
Fix leak of region reference that prevents complete
device deletion on hot unplug.

Cc: qemu-stable@nongnu.org
Signed-off-by: Yuri Benditovich <yuri.benditovich@daynix.com>
Message-Id: <20191226043649.14481-2-yuri.benditovich@daynix.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
(cherry picked from commit 421afd2fe8dd4603216cbf36081877c391f5a2a4)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>