tests: Drop qemuxml2argv- prefix for qemuxml2argv-*.xml test cases

tests: Drop qemuxml2argv- prefix for qemuxml2argv-*.xml test cases

These XMLs live in a separate directory, there's no need for them
to have a special prefix in addition. It also doesn't play nicely
with ':e' completion in Vim, finding proper file based on
qemuxml2argvtest.c is also needlessly complicated.

The files were renamed using the following commands. From
qemuxml2argvdata:

  for i in qemuxml2argv-*.xml; do mv $i ${i#qemuxml2argv-}; done

and then (to fix broken symlinks) from qemuxml2argvdata and
qemuxml2xmloutdata:

  for i in $(find . -xtype l); do \
      ln -sf $(readlink $i | sed 's/qemuxml2argv-//') $i;
  done

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>