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

src/Makefile.am

@@ -1143,7 +1143,7 @@ libvirt_driver_vbox_impl_la_CFLAGS =				\
 		-I$(top_srcdir)/src/conf			\
 		$(AM_CFLAGS)
 libvirt_driver_vbox_impl_la_LDFLAGS = $(AM_LDFLAGS)
-libvirt_driver_vbox_impl_la_LIBADD =  $(DLOPEN_LIBS) $(MSCOM_LIBS)
+libvirt_driver_vbox_impl_la_LIBADD =  $(DLOPEN_LIBS) $(MSCOM_LIBS) $(LIBXML_LIBS)
 libvirt_driver_vbox_impl_la_SOURCES = $(VBOX_DRIVER_SOURCES)
 endif WITH_VBOX