tests: Avoid use of virQEMUDriverCreateXMLConf(NULL)

tests: Avoid use of virQEMUDriverCreateXMLConf(NULL)

We use the function to create a virDomainXMLOption object that is
required for some functions. However, we don't pass the driver
pointer to the object anywhere - rather than pass NULL. This
causes trouble later when parsing a domain XML and calling post
parse callbacks:

  Program received signal SIGSEGV, Segmentation fault.
  0x000000000043fa3e in qemuDomainDefPostParse (def=0x7d36c0, caps=0x7caf10, opaque=0x0) at qemu/qemu_domain.c:1043
  1043        qemuCaps = virQEMUCapsCacheLookup(driver->qemuCapsCache, def->emulator);
  (gdb) bt
  #0  0x000000000043fa3e in qemuDomainDefPostParse (def=0x7d36c0, caps=0x7caf10, opaque=0x0) at qemu/qemu_domain.c:1043
  #1  0x00007ffff2928bf9 in virDomainDefPostParse (def=0x7d36c0, caps=0x7caf10, xmlopt=0x7c82c0) at conf/domain_conf.c:4269
  #2  0x00007ffff294de04 in virDomainDefParseXML (xml=0x7da8c0, root=0x7dab80, ctxt=0x7da980, caps=0x7caf10, xmlopt=0x7c82c0, flags=0) at conf/domain_conf.c:16400
  #3  0x00007ffff294e5b5 in virDomainDefParseNode (xml=0x7da8c0, root=0x7dab80, caps=0x7caf10, xmlopt=0x7c82c0, flags=0) at conf/domain_conf.c:16582
  #4  0x00007ffff294e424 in virDomainDefParse (xmlStr=0x0, filename=0x7c7ef0 "/home/zippy/work/libvirt/libvirt.git/tests/securityselinuxlabeldata/disks.xml", caps=0x7caf10, xmlopt=0x7c82c0, flags=0) at conf/domain_conf.c:16529
  #5  0x00007ffff294e4b2 in virDomainDefParseFile (filename=0x7c7ef0 "/home/zippy/work/libvirt/libvirt.git/tests/securityselinuxlabeldata/disks.xml", caps=0x7caf10, xmlopt=0x7c82c0, flags=0) at conf/domain_conf.c:16553
  #6  0x00000000004303ca in testSELinuxLoadDef (testname=0x53c929 "disks") at securityselinuxlabeltest.c:192
  #7  0x00000000004309e8 in testSELinuxLabeling (opaque=0x53c929) at securityselinuxlabeltest.c:313
  #8  0x0000000000431207 in virtTestRun (title=0x53c92f "Labelling \"disks\"", body=0x430964 <testSELinuxLabeling>, data=0x53c929) at testutils.c:211
  #9  0x0000000000430c5d in mymain () at securityselinuxlabeltest.c:373
  #10 0x00000000004325c2 in virtTestMain (argc=1, argv=0x7fffffffd7e8, func=0x430b4a <mymain>) at testutils.c:863
  #11 0x0000000000430deb in main (argc=1, argv=0x7fffffffd7e8) at securityselinuxlabeltest.c:381

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>