softmmu/vl.c: Check for the availability of the VGA device before using it

softmmu/vl.c: Check for the availability of the VGA device before using it

In case the user disabled the default VGA device in the binary (e.g.
with the "--without-default-devices" configure switch), we should
not try to use it by default if QEMU is running with the default
devices, otherwise it aborts when trying to use it. Simply emit a
warning instead.

Message-Id: <20230512124033.502654-3-thuth@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>