virsh: improve storage unit parsing

virsh: improve storage unit parsing

Now can now do:

virsh vol-resize $vol 10M
virsh blockresize $dom $vol 10M

to get both interfaces to resize to 10MiB.  The remaining wart
is that vol-resize defaults to bytes, but blockresize defaults
to KiB, but we can't break existing scripts; oh well, it's no
worse than the same wart of the underlying virDomainBlockResize.

The API for virStorageVolResize states that capacity must always
be positive, and that the presence of shrink and delta flags is
what implies a negative change.

* tools/virsh.c (vshCommandOptScaledInt): New function.
(cmdVolResize): Don't pass negative size.
(cmdVolSize): Rename...
(vshVolSize): ...and use new helper routine.
(cmdBlockResize): Use new helper routine, and support new bytes
flag.
* tools/virsh.pod (NOTES): Document suffixes.
(blockresize, vol-create-as, vol-resize): Point to notes.