tests: skip schema validation tests if xmllint is missing
tests: skip schema validation tests if xmllint is missing

On IRC, someone complained that a system without xmllint installed
failed a number of tests.

* tests/schematestutils.sh: Probe for xmllint.

Signed-off-by: Eric Blake <a9da18479c871fe903d9590822cfe2629c6a4d11@redhat.com>

jardasgit/libvirt,datto/libvirt,jardasgit/libvirt,taget/libvirt,datto/libvirt,nertpinx/libvirt,crobinso/libvirt,libvirt/libvirt,elmarco/libvirt,shugaoye/libvirt,elmarco/libvirt,VenkatDatta/libvirt,agx/libvirt,taget/libvirt,zippy2/libvirt,olafhering/libvirt,jardasgit/libvirt,eskultety/libvirt,andreabolognani/libvirt,datto/libvirt,agx/libvirt,datto/libvirt,andreabolognani/libvirt,zhlcindy/libvirt-1.1.4-maintain,VenkatDatta/libvirt,andreabolognani/libvirt,olafhering/libvirt,zhlcindy/libvirt-1.1.4-maintain,crobinso/libvirt,VenkatDatta/libvirt,jfehlig/libvirt,fabianfreyer/libvirt,eskultety/libvirt,shugaoye/libvirt,fabianfreyer/libvirt,libvirt/libvirt,datto/libvirt,taget/libvirt,zhlcindy/libvirt-1.1.4-maintain,nertpinx/libvirt,cbosdo/libvirt,VenkatDatta/libvirt,zippy2/libvirt,taget/libvirt,rlaager/libvirt,nertpinx/libvirt,cbosdo/libvirt,fabianfreyer/libvirt,crobinso/libvirt,zippy2/libvirt,jfehlig/libvirt,fabianfreyer/libvirt,rlaager/libvirt,crobinso/libvirt,nertpinx/libvirt,zhlcindy/libvirt-1.1.4-maintain,agx/libvirt,elmarco/libvirt,cbosdo/libvirt,zippy2/libvirt,rlaager/libvirt,shugaoye/libvirt,shugaoye/libvirt,rlaager/libvirt,olafhering/libvirt,nertpinx/libvirt,andreabolognani/libvirt,jardasgit/libvirt,andreabolognani/libvirt,shugaoye/libvirt,cbosdo/libvirt,VenkatDatta/libvirt,elmarco/libvirt,rlaager/libvirt,olafhering/libvirt,eskultety/libvirt,jfehlig/libvirt,cbosdo/libvirt,agx/libvirt,libvirt/libvirt,taget/libvirt,libvirt/libvirt,jardasgit/libvirt,elmarco/libvirt,eskultety/libvirt,agx/libvirt,eskultety/libvirt,jfehlig/libvirt,fabianfreyer/libvirt,zhlcindy/libvirt-1.1.4-maintain