tests: Fix up test numbering in qemuxml2argvtest

tests: Fix up test numbering in qemuxml2argvtest

Due to conditional execution of virTestRun(), the testCounter was incremented
only if all the cases were run.  When using VIR_TEST_RANGE=x-y, first x/2 of the
increments were skipped and that made figuring out a precise case a PITA.
Moving the condition into the test function makes it way nicer to find out the
test numbers to use in VIR_TEST_RANGE.

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>