caps: report if QEMU supports live snapshots

Depending on QEMU version and configuration, live snapshotting
may be not supported. In that case, a request sent by engine
will of course fail.

Until version 1.2.3, libvirt did not expose any information about
the live snapshot support, so VDSM, and then engine, had no
option other than assume this was supported.

This patch adds detection and reporting of QEMU live snapshot
support, so engine can make informed choices like, for example,
disable the relevant fields in the UI.

Change-Id: I78dd51fc72f1b6d7eadb5c18d3b768f42d8ee32b
Bug-Url: https://bugzilla.redhat.com/1009100
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/26149
Reviewed-by: Dan Kenigsberg <danken@redhat.com>