qemu: skip precreation of network disks

qemu: skip precreation of network disks

Commit cf54c60699833b3791a5d0eb3eb5a1948c267f6b introduced the ability
to create missing storage volumes during migration. For network disks,
however, we may not necessarily be able to detect whether they already
exist -- there is no straight-forward way to map the disk to a storage
volume, and even if there were it's possible no configured storage pool
actually contains the disk.

It is better to assume the network disk exists in this case, rather than
aborting the migration completely. If the volume really is missing, QEMU
will generate an appropriate error later in the migration.

Signed-off-by: Michael Chapman <mike@very.puzzling.org>