qemuDomainDeviceDefPostParse: Drop useless checks

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>