arm: vexpress64: juno: Allow boot from VirtIO

The AEM and Juno FVPs (Fixed Virtual Platforms) support a VirtIO
disc interface. Adding VIRTIO to the list of boot devices allows
these FastModel platforms to boot from 'disc' in the same way
the hardware counterpart can boot from SATA or USB.

This is a NOP if CONFIG_CMD_VIRTIO is not enabled, so no impact
on Juno hardware (which is built with vexpress_aemv8a_juno_defconfig)

Signed-off-by: Robert Catherall <robert.catherall@arm.com>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>