qemu: Don't add -spice port=0 when no port is specified

qemu: Don't add -spice port=0 when no port is specified

If a <graphics type='spice'> has no port nor tlsPort set, the generated
QEMU command line will contain -spice port=0.
This is later going to be ignored by spice-server, but it's better not
to add it at all in this situation.
As an empty -spice is not allowed, we still need to append port=0 if we
did not add any other argument.