vga: disable default VGA if appropriate -device is used

This is a partial revert of commits a369da5 (vga: improve VGA logic,
committed 2012-01-22) and c5bd4f3 (vga: fix -nodefaults -device VGA,
2012-01-24) which broke command-line option parsing in different ways.

Since commit a369da5 it has become impossible to specify a VGA device
entirely with QemuOpts-enabled options, i.e. without needing an explicit
"-vga none".

In addition, until commit c5bd4f3 -nodefaults would not disable the device
you specified with the legacy "-vga" option, independent of the order.
Since commit c5bd4f3 QEMU -nodefaults will override a previous -vga
option.

I did not reintroduce machine->no_vga.  Boards can simply ignore the
vga_interface_type variable, and most will indeed do so.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
