tests: utils: Fail XML file comparison if input file doesn't exist
tests: utils: Fail XML file comparison if input file doesn't exist

In cases where we expect parse failure of the test input file the
testsuite can't differentiate if the parser failed when parsing or when
opening the file. Add a call to virFileExists and error out on missing
input files.

Missing output files are partially expected when regenerating test
output.

jfehlig/libvirt,datto/libvirt,andreabolognani/libvirt,jfehlig/libvirt,datto/libvirt,zippy2/libvirt,nertpinx/libvirt,nertpinx/libvirt,datto/libvirt,eskultety/libvirt,VenkatDatta/libvirt,nertpinx/libvirt,crobinso/libvirt,jfehlig/libvirt,VenkatDatta/libvirt,jardasgit/libvirt,jardasgit/libvirt,eskultety/libvirt,crobinso/libvirt,datto/libvirt,jardasgit/libvirt,zippy2/libvirt,olafhering/libvirt,VenkatDatta/libvirt,andreabolognani/libvirt,jfehlig/libvirt,zippy2/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,jardasgit/libvirt,VenkatDatta/libvirt,andreabolognani/libvirt,taget/libvirt,crobinso/libvirt,olafhering/libvirt,datto/libvirt,taget/libvirt,eskultety/libvirt,olafhering/libvirt,libvirt/libvirt,olafhering/libvirt,nertpinx/libvirt,taget/libvirt,zippy2/libvirt,eskultety/libvirt,taget/libvirt,libvirt/libvirt,andreabolognani/libvirt,libvirt/libvirt,nertpinx/libvirt,VenkatDatta/libvirt,eskultety/libvirt,crobinso/libvirt,taget/libvirt,andreabolognani/libvirt,fabianfreyer/libvirt,fabianfreyer/libvirt,fabianfreyer/libvirt,fabianfreyer/libvirt,libvirt/libvirt