virsh-domif-setlink-getlink: add backup and restore of vm xml config

We need to backup and restore the guest xml config because we are
modifying the guest. If we don't do this, every other test will fail
because we break the network and the guest is not reachable by
ssh client.

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
Merge pull request #918 from Antique/virsh-domif-setlink-getlink

virsh-domif-setlink-getlink: add backup and restore of vm xml config