pseries: allow setting stdout-path even on machines with a VGA

pseries: allow setting stdout-path even on machines with a VGA

-machine graphics=off is the usual way to tell the firmware or the OS that the
user wants a serial console.  The pseries machine however does not support
this, and never adds the stdout-path node to the device tree if a VGA device
is provided.  This is in addition to the other magic behavior of VGA devices,
which is to add a keyboard and mouse to the default USB bus.

Split spapr->has_graphics in two variables so that the two behaviors can be
separated: the USB devices remains the same, but the stdout-path is added
even with "-device VGA -machine graphics=off".

Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20220507054826.124936-1-pbonzini@redhat.com>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>