remote_driver: Restore special behavior of remoteDomainGetBlockIoTune()

remote_driver: Restore special behavior of remoteDomainGetBlockIoTune()

In v9.10.0-rc1~103 the remote driver was switched to g_auto() for
client RPC return parameters. But whilst doing so a small bug
slipped in: previously, when virDomainGetBlockIoTune() was called
with *nparams == 0, the function set *nparams to the number of
supported params and zero was returned (so that client can
allocate memory and call the API second time). IOW - the usual,
old style of APIs where we didn't want to allocate memory on
caller's behalf. But because of this bug, a negative one is
returned instead.

Fixes: 501825011c1fe80f458820c7efe5a198e0af9be5
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>