blockdev: acquire AioContext in blockdev-snapshot-delete-internal-sync

Add dataplane support to the blockdev-snapshot-delete-internal-sync QMP
command.  By acquiring the AioContext we avoid race conditions with the
dataplane thread which may also be accessing the BlockDriverState.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
