virsh_migrate_setmaxdowntime: SKIP for undefined source URI

Follow virsh_migrate_stress to add 'virsh_migrate_src_uri' to manage the
source uri rather than depending on the 'src_uri' (which is None). Also
check if dest_uri and src_uri haven't changed from the default and SKIP
if so
Merge pull request #1240 from jferlan/virsh_migrate_setmaxdowntime

virsh_migrate_setmaxdowntime: SKIP for undefined source URI