When getting block device I/O tuning data there is no check for whether
QEMU supports such options and the call fails on
qemuMonitorGetBlockIoThrottle() when getting the particular throttle
data.  So try reporting a better error when blkdeviotune is not
supported.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1224053Signed-off-by: NMartin Kletzander <mkletzan@redhat.com>