conf: validate: Run global device definition validation before callbacks

conf: validate: Run global device definition validation before callbacks

The validation infrastructure doesn't modify the definition and
additionally it makes sense to run the global code first as it's
validating certain corner cases.

The changed error messages from qemuxml2argvtest show that this is
indeed the proper ordering as all changed messages are actually better
describing the error.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>