scsi: notify the device when unit attention is reported

Reporting media change events via unit attention sense codes requires
a small state machine: first report "NO MEDIUM", then report "MEDIUM MAY
HAVE CHANGED".  Unfortunately there is no good hooking point for the
device to notice that its pending unit attention condition has been
reported.  This patch reworks the generic machinery to add one.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>