virtio: Ensure virt_queue is always initialised

Failing to initialise the virt_queue via virtio_init_device_vq() leaves,
amongst other things, the endianness unspecified. On arm/arm64 this
results in virtio_guest_to_host_uxx() treating the queue as big-endian
and trying to translate bogus addresses:

  Warning: unable to translate guest address 0x80b8249800000000 to host

Ensure the virt_queue is always initialised by the virtio device during
setup.

Cc: Marc Zyngier <maz@kernel.org>
Cc: Julien Thierry <julien.thierry.kdev@gmail.com>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Tested-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>