vbox: fix linker error

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