[PV-ON-HVM] Fix evtchn of unbind_from_irqhandler()

[PV-ON-HVM] Fix evtchn of unbind_from_irqhandler()

When xm block-detach command was done on PV-ON-HVM, the response of
other disks was lost. It is because a wrong event channel was
invalidated when detaching it. Not the evtchn number but the irq
number is invalidated specifying it.

Signed-off-by: Takanori Kasai <kasai.takanori@jp.fujitsu.com>
Signed-off-by: Tomonari Horikoshi <t.horikoshi@jp.fujitsu.com>
Signed-off-by: Kazuhiro Suzuki <kaz@jp.fujitsu.com>