The previous OOM testing support would re-run the entire "main"
method each iteration, failing a different malloc each time.
When a test suite has 'n' allocations, the number of repeats
requires is  (n * (n + 1) ) / 2.  This gets very large, very
quickly.

This new OOM testing support instead integrates at the
virtTestRun level, so each individual test case gets repeated,
instead of the entire test suite. This means the values of
'n' are orders of magnitude smaller.

The simple usage is

   $ VIR_TEST_OOM=1 ./qemuxml2argvtest
   ...
   29) QEMU XML-2-ARGV clock-utc                                         ... OK
       Test OOM for nalloc=36 .................................... OK
   30) QEMU XML-2-ARGV clock-localtime                                   ... OK
       Test OOM for nalloc=36 .................................... OK
   31) QEMU XML-2-ARGV clock-france                                      ... OK
       Test OOM for nalloc=38 ...................................... OK
   ...

the second lines reports how many mallocs have to be failed, and thus
how many repeats of the test will be run.

If it crashes, then running under valgrind will often show the problem

  $ VIR_TEST_OOM=1 ../run valgrind ./qemuxml2argvtest

When debugging problems it is also helpful to select an individual
test case

  $ VIR_TEST_RANGE=30 VIR_TEST_OOM=1 ../run valgrind ./qemuxml2argvtest

When things get really tricky, it is possible to request that just
specific allocs are failed. eg to fail allocs 5 -> 12, use

  $ VIR_TEST_RANGE=30 VIR_TEST_OOM=1:5-12 ../run valgrind ./qemuxml2argvtest

In the worse case, you might want to know the stack trace of the
alloc which was failed then VIR_TEST_OOM_TRACE can be set. If it
is set to 1 then it will only print if it thinks a mistake happened.
This is often not reliable, so setting it to 2 will make it print
the stack trace for every alloc that is failed.

  $ VIR_TEST_OOM_TRACE=2 VIR_TEST_RANGE=30 VIR_TEST_OOM=1:5-5 ../run valgrind ./qemuxml2argvtest
  30) QEMU XML-2-ARGV clock-localtime                                   ... OK
      Test OOM for nalloc=36 !virAllocN
  /home/berrange/src/virt/libvirt/src/util/viralloc.c:180
  virHashCreateFull
  /home/berrange/src/virt/libvirt/src/util/virhash.c:144
  virDomainDefParseXML
  /home/berrange/src/virt/libvirt/src/conf/domain_conf.c:11745
  virDomainDefParseNode
  /home/berrange/src/virt/libvirt/src/conf/domain_conf.c:12646
  virDomainDefParse
  /home/berrange/src/virt/libvirt/src/conf/domain_conf.c:12590
  testCompareXMLToArgvFiles
  /home/berrange/src/virt/libvirt/tests/qemuxml2argvtest.c:106
  virtTestRun
  /home/berrange/src/virt/libvirt/tests/testutils.c:250
  mymain
  /home/berrange/src/virt/libvirt/tests/qemuxml2argvtest.c:418 (discriminator 2)
  virtTestMain
  /home/berrange/src/virt/libvirt/tests/testutils.c:750
  ??
  ??:0
  _start
  ??:?
   FAILED
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>