xen: xenbus PM events support

Make xenbus frontend device subscribe to PM events
to receive suspend/resume/freeze/thaw/restore
notifications.

Signed-off-by: Kenji Wakamiya <wkenji@jp.fujitsu.com>
Signed-off-by: Kazuhiro Suzuki <kaz@jp.fujitsu.com>
Signed-off-by: Shriram Rajagopalan <rshriram@cs.ubc.ca>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
[shriram--minor mods and improved commit message]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>