vga: improve VGA logic

Improve VGA selection logic, push check for device availabilty to vl.c.
Create the devices at board level unconditionally.

Remove now unused pci_try_create*() functions.

Make PCI VGA devices optional.

Reviewed-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>

hw/s390-virtio.c

@@ -317,7 +317,6 @@ static QEMUMachine s390_machine = {
     .no_serial = 1,
     .no_parallel = 1,
     .use_virtcon = 1,
-    .no_vga = 1,
     .max_cpus = 255,
     .is_default = 1,
 };