The default machine-type (pc-i440fx-2.0) now requires bios-256k.bin, but
"make install" isn't installing it, so qemu-system-x86_64 won't run out
of the box. Add it to BLOBS so it gets installed.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Tested-by: William Dauchy <william@gandi.net>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>