Fix /vm/uuid xenstore leak on tapdisk2 device cleanup

Fix /vm/uuid xenstore leak on tapdisk2 device cleanup

While doing block-detach blktap2 disk, tap2 device info is not cleared from
xenstore /vm/uuid/xxx. The reason is in xen-hotplug-cleanup script: when $vm_dev
does not exist, $(xenstore-read "$vm_dev" 2>/dev/null) is also "", won't enter
the block. So, change to use cmd return value to check existence.

Signed-off-by Chunyan Liu <cyliu@novell.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
committer: Stefano Stabellini <stefano.stabellini@eu.citrix.com>