iotests: clarify help text

Split the help text to highlight the groups of options
a little better, carving out a clear "format" and
"protocols" section.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-id: 20170427205100.9505-2-jsnow@redhat.com
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>

tests/qemu-iotests/common

@@ -138,7 +138,7 @@ common options
     -v                  verbose
     -d                  debug
 
-check options
+image format options
     -raw                test raw (default)
     -bochs              test bochs
     -cloop              test cloop
@@ -150,14 +150,18 @@ check options
     -vpc                test vpc
     -vhdx               test vhdx
     -vmdk               test vmdk
+    -luks               test luks
+
+image protocol options
     -file               test file (default)
     -rbd                test rbd
     -sheepdog           test sheepdog
     -nbd                test nbd
     -ssh                test ssh
     -nfs                test nfs
-    -luks               test luks
     -vxhs               test vxhs
+
+other options
     -xdiff              graphical mode diff
     -nocache            use O_DIRECT on backing file
     -misalign           misalign memory allocations