libvirt-tests: Add test for libvirt_bench_domstate_switch_in_loop

Test for the performance of libvirt in domain state switching.

Steps:
    * shutdown (or destroy)
    * start
    * suspend
    * resume

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
Merge pull request #806 from yangdongsheng/libvirt_bench

libvirt: add some tests for performance of libvirt.