Function qemuDomainSetBlockIoTune() was checking QEMU capabilities
even when !(flags & VIR_DOMAIN_AFFECT_LIVE) and the domain was
shutoff, resulting in the following problem:

 virsh # domstate asdf; blkdeviotune asdf vda --write-bytes-sec 100
 shut off

 error: Unable to change block I/O throttle
 error: unsupported configuration: block I/O throttling not supported with this QEMU binary

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=965016