Use automatic variable clearing and remove the cleanup sections of
testQemuDiskXMLToProps, testQemuDiskXMLToPropsValidateSchema and
testQemuDiskXMLToPropsValidateFile.

testQemuDiskXMLToPropsValidateFileSrcOnly already uses new helpers.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NMichal Privoznik <mprivozn@redhat.com>