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/alpha_pci.c b/hw/alpha_pci.c
index e975702..6735577 100644
--- a/hw/alpha_pci.c
+++ b/hw/alpha_pci.c
@@ -121,10 +121,8 @@
         pci_cirrus_vga_init(pci_bus);
         return;
     case VGA_VMWARE:
-        if (pci_vmsvga_init(pci_bus)) {
-            return;
-        }
-        break;
+        pci_vmsvga_init(pci_bus);
+        return;
     }
     /* If VGA is enabled at all, and one of the above didn't work, then
        fallback to Standard VGA.  */