Fix vga_interface_type for command line argument '-device VGA'

Some machine (like pseries) initialization code determines if it has
graphics according to vga_interface_type. In the original code,
vga_interface_type is evaluated to VGA_NONE even if a VGA is added
via '-device VGA'. It causes the machine not aware of the graphics
device configured. Add a new VGA device type to indicate that it has a
VGA device, which will be initialized in QOM device initialization.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Mark Wu <wudxw@linux.vnet.ibm.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
