The current priority of BOOTDEVP_4_SCAN_FAST means that virtio is
scanned after SCSI. But virtio-scsi provides higher performance.
It seems better to regard virtio has an internal device, since it is
paravirtualised and thus fast. Change its bootdev priority to
BOOTDEVP_2_INTERNAL_FAST
Disable the virtio-fs feature on sandbox, since the virtio queue is not
not yet supported by the emulator.
Signed-off-by: Simon Glass <sjg@chromium.org>