Add helpers for resolving path to resources in build tree

Add helpers for resolving path to resources in build tree

Add virFileFindResource which will try to locate files
in the local build tree if the calling binary (eg libvirtd or
test suite) is being run from the build tree. The corresponding
virFileActivateDirOverride should be called at startup passing
in argv[0]. This will be examined for evidence of libtool magic
binary prefix / sub-directory in order to activate the override.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>