virsh: Remove indirect link against libxml2

as described at
http://wiki.debian.org/ToolChain/DSOLinking
https://fedoraproject.org/wiki/UnderstandingDSOLinkChange

otherwise the build fails on current Debian unstable with:

CCLD   virsh
/usr/bin/ld: virsh-virsh.o: undefined reference to symbol 'xmlSaveTree@@LIBXML2_2.6.8'
/usr/bin/ld: note: 'xmlSaveTree@@LIBXML2_2.6.8' is defined in DSO //usr/lib/libxml2.so.2 so try adding it to the linker command line
//usr/lib/libxml2.so.2: could not read symbols: Invalid operation

tools/Makefile.am

@@ -47,6 +47,7 @@ virsh_LDADD =							\
 		../src/libvirt.la				\
 		../src/libvirt-qemu.la				\
 		../gnulib/lib/libgnu.la				\
+		$(LIBXML_LIBS)					\
 		$(VIRSH_LIBS)
 virsh_CFLAGS =							\
 		-I$(top_srcdir)/gnulib/lib -I../gnulib/lib	\