libvirt.tests: Add testcases for qemu monitor commands - blockjob.

For qemu-monitor-command block-job-*, now it supports copied migration.

Relative command:
block-job-cancel
block-job-complete
block-job-pause
block-job-resume

Rely on https://github.com/autotest/virt-test/pull/1810

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>