fsdev: Clean up error reporting in qemu_fsdev_add()

Calling error_report() from within a function that takes an Error **
argument is suspicious.  qemu_fsdev_add() does that, and its caller
fsdev_init_func() then fails without setting an error.  Its caller
main(), via qemu_opts_foreach(), is fine with it, but clean it up
anyway.

Cc: Greg Kurz <groug@kaod.org>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Acked-by: Greg Kurz <groug@kaod.org>
Message-Id: <20181017082702.5581-32-armbru@redhat.com>