This patch implements period and quota tunable XML configuration and parsing.
A quota or period of zero will be simply ignored.

Clang warned about a dead assignment.  In the process, I noticed
that we are only using the function for a bool value.  I audited
all other callers in qemu_{migration,cgroup,driver,hotplug), and
all were making the call in a bool context.

Also, do bounds checking on the argument.

* src/qemu/qemu_cgroup.c (qemuSetupCgroup): Delete dead
assignment.
(qemuCgroupControllerActive): Change return type to bool.
* src/qemu/qemu_cgroup.h (qemuCgroupControllerActive): Likewise.

* src/qemu/qemu_cgroup.h (struct qemuCgroupData): New helper type.
(qemuSetupDiskPathAllow, qemuSetupChardevCgroup)
(qemuTeardownDiskPathDeny): Drop unneeded prototypes.
(qemuSetupDiskCgroup, qemuTeardownDiskCgroup): Adjust prototype.
* src/qemu/qemu_cgroup.c
(qemuSetupDiskPathAllow, qemuSetupChardevCgroup)
(qemuTeardownDiskPathDeny): Mark static and use new type.
(qemuSetupHostUsbDeviceCgroup): Use new type.
(qemuSetupDiskCgroup): Alter signature.
(qemuSetupCgroup): Adjust caller.
* src/qemu/qemu_hotplug.c (qemuDomainAttachHostUsbDevice)
(qemuDomainDetachPciDiskDevice, qemuDomainDetachSCSIDiskDevice):
Likewise.
* src/qemu/qemu_driver.c (qemudDomainAttachDevice)
(qemuDomainUpdateDeviceFlags): Likewise.

The QEMU driver file is far too large. Move all the cgroup
helper code out into a separate file. No functional change.

* src/qemu/qemu_cgroup.c, src/qemu/qemu_cgroup.h,
  src/Makefile.am: Add cgroup helper file
* src/qemu/qemu_driver.c: Delete cgroup code