init/do_mounts.c: add virtiofs root fs support

Make it possible to boot directly from a virtiofs file system with tag
'myfs' using the following kernel parameters:

  rootfstype=virtiofs root=myfs rw

Booting directly from virtiofs makes it possible to use a directory on
the host as the root file system.  This is convenient for testing and
situations where manipulating disk image files is cumbersome.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>