The current state of virsh parsing is that:

$ virsh vol-info /path/to/image
$ virsh vol-info --pool default /path/to/image
$ virsh vol-info --pool default --vol /path/to/image

all lookup the volume by path (technically, the last two also attempt
a name lookup within a pool, whereas the first skips that step, but
the end result is the same); meanwhile:

$ virsh vol-info default /path/to/image

complains about unexpected data.  Why?  Because the --pool option is
optional, so default was parsed as the --vol argument, and
/path/to/image.img doesn't match up with any remaining options that
require an argument.  For proof, note that:

$ virsh vol-info default --vol /path/to/image

complains about looking up 'default' - the parser mis-associated both
arguments with --vol.  Given the above, the only way to specify pool
is with an explicit "--pool" argument (you can't specify it
positionally).  However, named arguments can appear in any order, so:

$ virsh vol-info /path/to/image --pool default
$ virsh vol-info --vol /path/to/image --pool default

have also always worked.  Therefore, this patch has no functional
change on vol-info option parsing, but only on 'virsh help vol-info'
synopsis layout.  However, it also allows the next patch to 1) enforce
that required options are always first (without this patch, the next
patch would fail the testsuite), and 2) allow the user to omit the
"--pool" argument.  That is, the next patch makes it possible to do:

$ virsh vol-info /path/to/image default

which to date was not possible.

* tools/virsh.c (opts_vol_create_from, opts_vol_clone)
(opts_vol_upload, opts_vol_download, opts_vol_delete)
(opts_vol_wipe, opts_vol_info, opts_vol_dumpxml, opts_vol_key)
(opts_vol_path): List optional pool parameter after required
arguments.