vl: Simplify machine_parse()

Exploit that argument @name is nerver null.  Check is_help_option()
first, because that's what we do elsewhere.  If we (foolishly!)
defined a machine named "help", -machine help would now print help
instead of selecting the machine named "help".

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Wei Yang <richardw.yang@linux.intel.com>
Message-Id: <20190405064121.23662-5-richardw.yang@linux.intel.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>