Cleanup shutdown cancel path after use

Since private xenstore paths are now UUID-based, a shutdown cancel path persist
across a localhost migration. This causes a following localhost migration to
fail: the cancel path that still exists causes the cancellable watch that waits
for the domain to ack the shutdown request to fire immediately, and cancel the
migration.

We solve it by cleaning up the path immediately after writing it. Since any
relevant cancellable watches would have already fired, there is no need to keep
it.

Signed-off-by: Rob Hoes <rob.hoes@citrix.com>