iotests: fix exclusion option

If you are running out-of-tree, the -x option to exclude
a certain iotest is broken.

Replace porcelain usage of ls with a sturdier awk command.
Reviewed-by: NFam Zheng <famz@redhat.com>
Signed-off-by: NJohn Snow <jsnow@redhat.com>
Message-id: 20170427205100.9505-3-jsnow@redhat.com
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NMax Reitz <mreitz@redhat.com>

tests/qemu-iotests/common

@@ -86,7 +86,8 @@ s/ .*//p
     elif $xgroup
     then
         # arg after -x
-        [ ! -s $tmp.list ] && ls [0-9][0-9][0-9] [0-9][0-9][0-9][0-9] >$tmp.list 2>/dev/null
+        # Populate $tmp.list with all tests
+        awk '/^[0-9]{3,}/ {print $1}' "${source_iotests}/group" > $tmp.list 2>/dev/null
         group_list=`sed -n <"$source_iotests/group" -e 's/$/ /' -e "/^[0-9][0-9][0-9].* $r /"'{
 s/ .*//p
 }'`