libxl: HVM domain S3 bugfix

libxl: HVM domain S3 bugfix

Currently Xen hvm s3 has a bug coming from the difference between
qemu-traditional and qemu-xen. For qemu-traditional, the way to
resume from hvm s3 is via 'xl trigger' command. However, for
qemu-xen, the way to resume from hvm s3 inherited from standard
qemu, i.e. via QMP, and it doesn't work under Xen.

The root cause is, for qemu-xen, 'xl trigger' command didn't reset
devices, while QMP didn't unpause hvm domain though they did qemu
system reset.

We have two qemu patches one xl patch to fix the HVM S3 bug:
This patch is the xl patch. It invokes QMP system_wakeup so that
qemu logic for hvm s3 could be triggered.

Signed-off-by: Liu Jinsong <jinsong.liu@intel.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>