virt-top: correct "virt-top -1" command via cpuacct cgroup

virt-top: correct "virt-top -1" command via cpuacct cgroup

Old "virt-top -1" is not correct, its output is generated by guess:
use average usage for pinned physical CPUs.

example(old "virt-top -1"):

PHYCPU %CPU rhel6  Windows
   0    0.6  0.1=   0.5=
   1    0.6  0.1=   0.5=#
   2    0.6  0.1=   0.5=
   3    0.6  0.1=#  0.5=

The output almost makes no sense(all the value are just average, not real).

This is new implement, it use cpuacct cgroup to gain *real* physical usages
via cpuacct cgroup by virDomainGetCPUStats() API.

new result:

PHYCPU %CPU rhel6  Windows
   0    1.3  0.3     1.0
   1    2.3  0.3     2.0
   2    2.2  0.5     1.7
   3    2.5  0.4     2.1

PHYCPU %CPU rhel6  Windows
   0    1.7  0.4     1.3
   1    3.6  1.0     2.7
   2    1.6  0.4     1.2
   3    4.8  3.1     1.7

Note: average flag(=) is dropped, there is not average value in here.
Note: running flag(#) is dropped, because if the value is not empty,
      it means the guest was once running in the physical CPU in this period
      between updates.

Changed from V3:
	use new virDomainGetCPUStats() libvirt-API.
	add a new function find_usages_from_stats() to gain cpu usages.

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