qemu: prefer .requiredCaps for VIR_DOMAIN_STATS_IOTHREAD

qemu: prefer .requiredCaps for VIR_DOMAIN_STATS_IOTHREAD

Since f29d7c3e698 we have an option for checking capabilities
required for given type of statistics upfront, instead of the
callback. Switch qemuDomainGetStatsIOThread() callback to the new
style.

This will now error out properly if user requests IOTHREAD stats
forcibly (via VIR_CONNECT_GET_ALL_DOMAINS_STATS_ENFORCE_STATS
flag) but QEMU doesn't support IOThreads. Previously, this was
silently ignored.

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