conf: report errors when parsing video resolution

conf: report errors when parsing video resolution

The current code doesn't properly handle errors when parsing a video
device's resolution.  We were returning a NULL structure for the case
where 'x' or 'y' were missing. But for the other error cases, we were
logging an error (virReportError()), but still returning an
under-specified structure. That under-specified structure was used by
the calling function rather than properly reporting an error.

This patch changes the parse function to return NULL on any parsing
error and changes the calling function to report an error when NULL is
returned.

Reviewed-by: Cole Robinson <crobinso@redhat.com>
Signed-off-by: Jonathon Jongsma <jjongsma@redhat.com>