testCompareXMLToArgv: Split out preparation and command formatting

testCompareXMLToArgv: Split out preparation and command formatting

There are multiple steps of setting up the domain definition prior to
formatting the command line for the tests. Extract it to a separate
function so that it's self-contained and also will allow re-running the
command line formatting which will be necessary for QMP schema
validation tests.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>