Make sure we only assign the default spicevmc channel name to spicevmc
virtio channels. Caused by commits 3269ee65 and 1133ee2b, which moved
the assignment from XML parsing code to QEMU but failed to keep the
logic.

https://bugzilla.redhat.com/show_bug.cgi?id=1179680Signed-off-by: NJiri Denemark <jdenemar@redhat.com>