Make QEMU sanity check a build blocker
If it ever breaks, it can be skipped by setting the %qemu_sanity_check
variable to 0.
This ensures it does not bit-rot again in future and actually adds some
value to the build.
It has to be skipped for ARM for now due to inability to select the
machine type.
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>