Add --nocompat option to disable compat warnings

Commit e6694207 ("kvm tools: Guest kernel compatability") added the
functionality that enables devices to print a warning message if the device
hasn't been initialized by the time the VM is destroyed. The purpose of
these messages is to let the user know if the kernel hasn't been built with
the correct Kconfig options to take advantage of the said devices (all
using virtio).

Since then, kvmtool has evolved and now supports loading different payloads
(like firmware images), and having those warnings even when it is entirely
intentional for the payload not to touch the devices can be confusing for
the user and makes the output unnecessarily verbose in those cases.

Add the --nocompat option to disable the warnings; the warnings are still
enabled by default.

Reported-by: Christoffer Dall <christoffer.dall@arm.com>
Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>