Minor cleanup made possible by previous simplifications.

* src/qemu/qemu_cgroup.h (qemuSetupDiskCgroup)
(qemuTeardownDiskCgroup): Alter signature.
* src/qemu/qemu_cgroup.c (qemuSetupDiskCgroup)
(qemuTeardownDiskCgroup, qemuSetupCgroup): Update all uses.
* src/qemu/qemu_hotplug.c (qemuDomainDetachPciDiskDevice)
(qemuDomainDetachDiskDevice): Likewise.
* src/qemu/qemu_driver.c (qemuDomainAttachDeviceDiskLive)
(qemuDomainChangeDiskMediaLive)
(qemuDomainSnapshotCreateSingleDiskActive)
(qemuDomainSnapshotUndoSingleDiskActive): Likewise.