Introduce a compilation option to disable strict enforcement of compliance
against VirtIO Version_1 backends, so as to allow to support also Legacy
VirtIO devices implementations.

Link: https://lore.kernel.org/r/20211115102910.7639-1-cristian.marussi@arm.comSigned-off-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>