qemu: switch s390/s390x default console back to serial

qemu: switch s390/s390x default console back to serial

Now that <serial> and <console> on s390/s390x behave a bit more like the
other architectures, remove this extra differentation, and use sclp
console by default for new guests.  New virtio consoles can still be
added, and it is actually needed because of the limited number of
instances for sclp and sclplm.

This reverts commit b1c88c14764e0b043a269d454a83a6ac7af34eac, whose
reasons are not totally clear.

Signed-off-by: Pino Toscano <ptoscano@redhat.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Bjoern Walk <bwalk@linux.vnet.ibm.com>