qemu: hotplug: Properly clean up drive backend if frontend hotplug fails

qemu: hotplug: Properly clean up drive backend if frontend hotplug fails

Commit 8125113c added code that should remove the disk backend if the
fronted hotplug failed for any reason. The code had a bug though as it
used the disk string for unplug rather than the backend alias. Fix the
code by pre-creating an alias string and using it instead of the disk
string. In cases where qemu does not support QEMU_CAPS_DEVICE, we ignore
the unplug of the backend since we can't really create an alias in that
case.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1262399