virtio: initialize earlier

Move initialization of the virtio framework before the initialization of
mtd, so that block2mtd can be used on virtio-based block devices.

Addresses https://bugzilla.kernel.org/show_bug.cgi?id=15644

Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>