Fix release_drive on unplugged devices (pci_piix3_xen_ide_unplug)

pci_piix3_xen_ide_unplug should completely unhook the unplugged
IDEDevice from the corresponding BlockBackend, otherwise the next call
to release_drive will try to detach the drive again.

Suggested-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>