softmmu/vl: Fence 'xenfb' if Xen support is not compiled in

softmmu/vl: Fence 'xenfb' if Xen support is not compiled in

The 'xenfb' parameter for the '-vga' command line option is currently
always enabled unconditionally (since the xenfb is not a proper QOM
device that could be tested via its class name). That means it also
shows up if Xen is not enabled at all, e.g. like this:

 $ ./qemu-system-sparc -vga help
 none                 no graphic card
 xenfb                Xen paravirtualized framebuffer
 tcx                  TCX framebuffer (default)
 cg3                  CG3 framebuffer

Let's avoid this situation by fencing the parameter with the
CONFIG_XEN_BACKEND switch.

Message-Id: <20220427123316.329312-1-thuth@redhat.com>
Acked-by: Anthony PERARD <anthony.perard@citrix.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>