This option, when added via
  TESTS_ENVIRONMENT = [...] $(top_builddir)/run --test
allows us to run the tests and only print the full output (including
debugging etc) when the test fails.

Old <libvirt.h> had a conflicting definition of virDomainGetXMLDesc:

fake_libvirt_xml.c:36: error: conflicting types for 'virDomainGetXMLDesc'
/usr/include/libvirt/libvirt.h:715: error: previous declaration of 'virDomainGetXMLDesc' was here

The difference is not material ('int' vs. 'unsigned int').

Avoid the error by not including <libvirt.h>.

RHEL 5-era autoconf did not define these, so define them manually
when they are missing.

Define builddir as '.'  The scripts require this.  It won't work
in the srcdir != builddir case, but we don't care about that for
RHEL 5.

This commit also moves the builddir / abs_srcdir variable setting
above the include of subdir-rules.mk, in case that include uses
these variables.

Useful script:

  for f in $(find -name Makefile.am | xargs fgrep '$(abs_srcdir)' -l) ; do
    if ! grep -q '^abs_srcdir' $f; then
      echo missing in $f
    fi
  done

This commit adds a tests/xml directory, and an LD_PRELOAD module which
can fake arbitrary libvirt XML from an external file (and is therefore
a much more flexible test than using the libvirt test:// driver
alone).

Also added is one regression test for:
https://bugzilla.redhat.com/show_bug.cgi?id=701814

Loading the given libvirt XML using Sys::Guestfs::Lib::open_guest
used to fail with the error:

  format parameter is empty or contains disallowed characters at /home/rjones/d/libguestfs/perl/blib/lib/Sys/Guestfs/Lib.pm line 256.

Thanks to Tom Horsley for supplying the test data.