vm: more generic recovery check

On recovery, VDSM checks each VM listed by libvirt
to see if it should take care of it.
The check is done by using some system properties
added in the smbios domain element, which is lacking
outside x86_64.

As result, recovery finds no VMs on PPC64.

This patch makes VDSM use the vm guest agent channels
to do the recovery check, since they are avaialble
on all plataforms.

Change-Id: Ic0cc57129e9c8e6545f9a947329adf1f9e82648f
Bug-Url: https://bugzilla.redhat.com/1126887
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/31241
Reviewed-by: Dan Kenigsberg <danken@redhat.com>