fix rpmbuild error

nbdkit.spec

@@ -4,7 +4,7 @@
 
 Name:           nbdkit
 Version:        1.6.2
-Release:        6
+Release:        7
 Summary:        NBD server
 License:        BSD
 URL:            https://github.com/libguestfs/nbdkit
@@ -319,6 +319,7 @@ rm $RPM_BUILD_ROOT%{_mandir}/man1/nbdkit-vddk-plugin.1*
 mkdir -p $HOME/.cache/libvirt
 export LIBGUESTFS_DEBUG=1
 export LIBGUESTFS_TRACE=1
+export LIBGUESTFS_BACKEND=direct
 make check -j1 || {
     cat tests/test-suite.log
     exit 1
@@ -422,6 +423,9 @@ popd
 %{_mandir}/man3/nbdkit-*.3*
 
 %changelog
+* Wed Dec 8 2021 caodongxia <caodongxia@huawei.com> - 1.6.2-7
+- fix rpmbuild error
+
 * Fri May 28 2021 wutao <wutao61@huawei.com> - 1.6.2-6
 - fix test error