Use kernel common header for fw_cfg.

(unfortunately, optionrom.h must have its own define, since it's
actually an assembler header)

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20180817155910.5722-2-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>