Long lines are harder to read and harder to diff; in fact, if lines get
too long (> 1000 bytes), it starts causing issues where git send-email
refuses to send patches for the file.  I've cleaned up the tests
directory in the past (see commits bd6c46fa, 3b750d13), but new long
lines have been introduced in the meantime.

Why 90 instead of 80? Because there were too many tests on the fringe
edge, and I didn't want to edit that many files.

Add a syntax check to prevent future long lines.

* cfg.mk (sc_prohibit_long_lines): New rule.
* tests/qemuxml2argvdata/qemuxml2argv-*.args: Split lines of any
file with content longer than 90 columns.
* tests/storagevolxml2argvdata/*.argv: Likewise.
Signed-off-by: NEric Blake <eblake@redhat.com>