tests: xmconfigtest: add tests for cmdline formating

Commit 656151bf fixed formatting of the <cmdline> element. Perhaps it
would have been noticed and fixed earlier if we had a test. With this
change, all possible cases of formatting <cmdline> from xmconfig are
covered

1. no 'extra=' or 'root=' in xm.cfg
2. 'extra=' but no 'root=' in xm.cfg
3. 'root=' but no 'extra=' in xm.cfg
4. both 'root=' and 'extra=' in xm.cfg

Case 1 is covered by all existing paravirt tests since they have no
'extra=' or 'root='. Case 2 is covered by adding 'extra=' to a few
of the existing paravirt tests. Cases 3 and 4 are covered by new
tests that only test conversion of xm.cfg to xml.

Signed-off-by: Jim Fehlig <jfehlig@suse.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>