qemu: reject blockiotune if qemu too old

qemu: reject blockiotune if qemu too old

Commit 4c82f09e added a capability check for qemu per-device io
throttling, but only applied it to domain startup.  As mentioned
in the previous commit (98cec05), the user can still get an 'internal
error' message during a hotplug attempt, when the monitor command
doesn't exist.  It is confusing to allow tuning on inactive domains
only to then be rejected when starting the domain.

* src/qemu/qemu_driver.c (qemuDomainSetBlockIoTune): Reject
offline tuning if online can't match it.