MFC r231883

Fix regression in the handling of blkback close events for
devices that are unplugged via QEMU.

sys/dev/xen/blkback/blkback.c:
Toolstack initiated closures change the frontend's state
to Closing. The backend must change to Closing as well,
even if we can't actually close yet, in order for the
frontend to notice and start the closing process.