examples: Try harder to uninstall nwfilter

examples: Try harder to uninstall nwfilter

We have this code in our Makefile that tries to remove
/etc/libvirt/nwfilter if directory is left empty after all our
example nwfilters were uninstalled. However, the check for that
is missing quotation marks thus rendering the test useless:

test -z allow-arp.xml allow-dhcp-server.xml .. qemu-announce-self.xml || \
  rmdir "/some/path/libvirt.git/_install/etc/libvirt/nwfilter"
/bin/sh: line 0: test: too many arguments

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>