tests: Enable directory override for qemucapsprobe

tests: Enable directory override for qemucapsprobe

Currently, qemucapsprobe fails when libvirt is not already installed
on the system:

  $ ./tests/qemucapsprobe /path/to/qemu-system-ppc64 >/dev/null
  I/O warning : failed to load external entity "/usr/share/libvirt/cpu_map/index.xml"
  2020-05-06 09:49:59.136+0000: 269822: info : libvirt version: 6.4.0
  2020-05-06 09:49:59.136+0000: 269822: info : hostname: [...]
  2020-05-06 09:49:59.136+0000: 269822: warning : virQEMUCapsLogProbeFailure:5127 :
  Failed to probe capabilities for /path/to/qemu-system-ppc64: XML error: failed to
  parse xml document '/usr/share/libvirt/cpu_map/index.xml'

It would be great if the tool could work entirely out of the build
directory, and this patch achieves just that.

Suggested-by: Peter Krempa <pkrempa@redhat.com>
Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>