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>
diff --git a/hw/qdev.h b/hw/qdev.h
index 2abb767..6b58dd8 100644
--- a/hw/qdev.h
+++ b/hw/qdev.h
@@ -179,6 +179,7 @@
 
 DeviceState *qdev_create(BusState *bus, const char *name);
 DeviceState *qdev_try_create(BusState *bus, const char *name);
+bool qdev_exists(const char *name);
 int qdev_device_help(QemuOpts *opts);
 DeviceState *qdev_device_add(QemuOpts *opts);
 int qdev_init(DeviceState *dev) QEMU_WARN_UNUSED_RESULT;