virschematest: Link with libxml2

virschematest: Link with libxml2

We use libxml2 APIs in the test (e.g. xmlFreeDoc) but not link
with -lxml2 which can cause problems:

/usr/bin/ld: virschematest.o: undefined reference to symbol 'xmlFreeDoc@@LIBXML2_2.4.30'
//usr/lib/x86_64-linux-gnu/libxml2.so.2: error adding symbols: DSO missing from command line
collect2: error: ld returned 1 exit status
Makefile:4702: recipe for target 'virschematest' failed

Reported-by: Katerina Koukiou <k.koukiou@googlemail.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>