tools: unlink bash completion files before symlinking

"ln" will not replace an existing symlink, so if you run 'make install'
twice, the second time will get an error:

  ln: failed to create symbolic link 'virsh': File exists

We must always remove the symlink target first.
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>

tools/Makefile.am

@@ -425,6 +425,7 @@ install-bash-completion:
 	$(INSTALL_SCRIPT) $(srcdir)/bash-completion/vsh \
 		"$(DESTDIR)$(BASH_COMPLETIONS_DIR)/vsh"
 	( cd $(DESTDIR)$(BASH_COMPLETIONS_DIR) && \
+		rm -f virsh virt-admin && \
 		$(LN_S) vsh virsh && \
 		$(LN_S) vsh virt-admin )