Toolstack::libvirt: Support for ACPI fallback for shutdown

Toolstack::libvirt: Support for ACPI fallback for shutdown

This is the libvirt counterpart to "Toolstack::xl: Support for ACPI
fallback for shutdown". Currently there are no jobs which test HVM
guests with libvirt and so this is completely untested in the context
of osstest (but at least should be harmless to current jobs).

This relies on an assumption that "virsh shutdown" behaves the same as
"virsh reboot" and accepts a comma separated list of methods to try
given to the --mode argument, which Jim has tested and confirmed to be
true.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Jim Fehlig <jfehlig@suse.com>
Acked by: Jim Fehlig <jfehlig@suse.com>