Some VGA adapters do not contain an helpful description,
this can be confusing:

  $ qemu-system-arm -M virt -vga help
  none
  std                  standard VGA
  cirrus               Cirrus VGA (default)
  vmware               VMWare SVGA
  xenfb

Add a description to the missing adapters:

  $ qemu-system-arm -M virt -vga help
  none                 no graphic card
  std                  standard VGA
  cirrus               Cirrus VGA (default)
  vmware               VMWare SVGA
  xenfb                Xen paravirtualized framebuffer

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Based-on: <20190412152713.16018-1-marcandre.lureau@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>
Message-Id: <20190412163706.3878-1-philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>