libvirtvm: avoid concurrent VM changes during saveState

When multiple changes of the VM configuration are applied at the
same time there's an high chance of saveState failing during a
deepcopy for a RuntimeError exception (e.g. dictionary changed size
during iteration). To avoid this problem the configuration changes
and the saveState requests are now serialized.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=948940
Change-Id: Ic08b4073f5e3f5184baa5f1c7dd3ec5a148ff60b
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/13624
Tested-by: Daniel Paikov <paikov@gmail.com>
Reviewed-by: Ayal Baron <abaron@redhat.com>