virttest: new test-case for qemu drive_mirror feature

New test case for qemu drive-mirror feature, implement support to check 
1: Cancel live mirroring
2: Query live mirroring job status
3: Set live mirroring speed limit
4: Live mirroring while guest reboot
5: Stop guest during live mirroring
6: Power down during live mirroring steady state
7: Live mirroring when guest under stress
Know Issue:
Third-parted test tools used in test (eg, stress, heavyload, dd for windows), script will download them from Internet, but this links not always available so,  it's better to download them into a internal repo to ensure test run stably;

Signed-off-by: Xu Tian <xutian@redhat.com>