svirt_start_destroy: Add test cases for netfs volume

With virt_use_nfs off when selinux enforcing, domain with image on
nfs will fail to start, domain could start while virt_use_nfs on.

Attach netfs disk to domain and set virt_use_nfs, then testing start
domain.

Signed-off-by: Wayne Sun <gsun@redhat.com>