block/rbd: don't copy strings in qemu_rbd_next_tok()

block/rbd: don't copy strings in qemu_rbd_next_tok()

This patch is prep work for parsing options for .bdrv_parse_filename,
and using QDict options.

The function qemu_rbd_next_tok() searched for various key/value pairs,
and copied them into buffers.  This will soon be an unnecessary extra
step, so we will now return found strings by reference only, and
offload the responsibility for safely handling/coping these strings to
the caller.

This also cleans up error handling some, as the callers now rely on
the Error object to determine if there is a parse error.

Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Jeff Cody <jcody@redhat.com>