e820: Add E820_SOFT_RESERVED type support

Recognize E820 type 0xEFFFFFFF (Soft Reserved) emitted by QEMU and
propagate it to the guest E820 map.  Without this, SeaBIOS would
silently drop the entry and the guest kernel could not honor the
reservation.

Signed-off-by: FangSheng Huang <FangSheng.Huang@amd.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>