Qemu will be the first driver to make use of a typed string in the
next round of additions.  Separate out the trivial addition.

* src/qemu/qemu_driver.c (qemudSupportsFeature): Advertise feature.
(qemuDomainGetBlkioParameters, qemuDomainGetMemoryParameters)
(qemuGetSchedulerParametersFlags, qemudDomainBlockStatsFlags):
Allow typed strings flag where trivially supported.