virsh: Remove indirect link against libxml2

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