vbox: fix linker error
    
    Noticed the following error when building the vbox driver
    in the openSUSE build service
    
    CCLD     vboxsnapshotxmltest
    /usr/lib64/gcc/x86_64-suse-linux/4.8/../../../../x86_64-suse-linux/bin/ld:
    ../src/.libs/libvirt_driver_vbox_impl.a
    (libvirt_driver_vbox_impl_la-vbox_snapshot_conf.o):
    undefined reference to symbol 'xmlXPathRegisterNs@@LIBXML2_2.4.30'
    /usr/lib64/libxml2.so: error adding symbols: DSO missing from command line
    collect2: error: ld returned 1 exit status
    
    Fixed by adding LIBXML_LIBS to libvirt_driver_vbox_impl_la_LIBADD