block: cancel jobs when a device is ready to go away

block: cancel jobs when a device is ready to go away

We do not want jobs to keep a device busy for a possibly very long
time, and management could become confused because they thought a
device was not even there anymore.  So, cancel long-running jobs
as soon as their device is going to disappear.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>