qemu: Implement cfs_period and cfs_quota's modification

qemu: Implement cfs_period and cfs_quota's modification

This patch implements cfs_period and cfs_quota's modification.
We can use the command 'virsh schedinfo' to query or modify cfs_period and
cfs_quota.
If you query period or quota from config file, the value 0 means it does not set
in the config file.
If you set period or quota to config file, the value 0 means that delete current
setting from config file.
If you modify period or quota while vm is running, the value 0 means that use
current value.