libvirt: Add virsh blockpull commandline testcase

New files: domain/virsh_blockpull.cfg|py
New functions in virttest/virsh.py: blockpull()

This pullreq is better be reviewed with testcase virsh_blockcommit.cfg|py together
https://github.com/autotest/virt-test/pull/1396

The testcase used a common helper function get_first_disk_devices which is defined in the commit of blockcommit pullreq