xend: Fix removing /vm/UUID/device paths when device cannot be disconnected

xend: Fix removing /vm/UUID/device paths when device cannot be disconnected

This is a corrected version of a patch commited as c/s 19250 and
reverted by c/s 19314. Thanks to "trap sigerr ERR" in
xen-hotplug-common.sh the xen-hotplug-cleanup would exit when reading
/local/domain/ID/vm fails thus skipping all the xenstore-rm lines in
the rest of the script.

Change deviceDestroy behavior to remove /vm/UUID/device/...
path only when force was used (as it already does so for both frontend
and backend) and do the removing from xen-hotplug-cleanup script when
we are sure the device is really not attached to the guest any more.

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>