engine: virtio -> VirtIO

This patch correct wrong VirtIO case.

Change-Id: Ifc2b02e02f1656b63dba84ffbe998f4a1db8a0a3
Bug-Url: https://bugzilla.redhat.com/999977
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>