qemu: switch to use -M graphics=off instead of -device sga

qemu: switch to use -M graphics=off instead of -device sga

SeaBIOS >= 1.11 has built-in support for outputting to the serial
console when QEMU sets -M graphics=off. Our minimum QEMU version
is 2.11.0, which bundled SeaBIOS 1.11. Thus we have no need to
use '-device sga' anymore.

This change results in a slight layout difference for option ROMs
in memory, however, it does not affect the migration data stream
format on the wire and once migration is complete the target QEMU
memory layout for ROMs matches the source QEMU once again.

Reviewed-by: Ján Tomko <jtomko@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>