qemu: Regenerate VNC socket paths

qemu: Regenerate VNC socket paths

Similarly to what commit 714080791778 did with some internal paths,
clear vnc socket paths that were generated by us.  Having such path in
the definition can cause trouble when restoring the domain.  The path is
generated to the per-domain directory that contains the domain ID.
However, that ID will be different upon restoration, so qemu won't be
able to create that socket because the directory will not be prepared.

To be able to migrate to older libvirt, skip formatting the socket path
in migratable XML if it was autogenerated.  And mark it as autogenerated
if it already exists and we're parsing live XML.

Best viewed with '-C'.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1326270

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>