qemuDomainDeviceDefPostParse: Drop useless checks
    
    Now that virQEMUDriverCreateXMLConf is never called with NULL
    (after 086f37e97aab) we can safely drop useless check in
    qemuDomainDeviceDefPostParse as we are guaranteed to be always
    called with the driver initialized. Therefore checking if driver
    is NULL makes no sense. Moreover, if we mix it with direct driver
    dereference. And after that, we are sure that nor @cfg will be
    NULL, therefore we can drop checks for that too.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>