qemu_driver: add virCgroupMounted

qemu_driver: add virCgroupMounted

When we mount any cgroup without "-o devices", we will fail to start vms:

error: Failed to start domain vm1
error: Unable to deny all devices for vm1: No such file or directory

When we mount any cgroup without "-o cpu", we will fail to get schedinfo:
Scheduler      : posix
error: unable to get cpu shares tunable: No such file or directory

We should only use the cgroup controllers which are mounted on host.
So I add virCgroupMounted() for qemuCgroupControllerActive()

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>