Commits 4bc42986 and 218c81ea removed virDomainStorageSourceFormat on
the grounds that there were no external callers; however, the upcoming
backup code wants to output a <target> (push mode) or <scratch> (pull
mode) element that is in all other respects identical to a domain's
<source> element, where the previous virDomainStorageSourceFormat fit
the bill nicely. But rather than reverting the commits, it's easier to
just add an additional parameter for the element name to use, and
update all callers.
Signed-off-by: NEric Blake <eblake@redhat.com>