virttest.virsh: Add new blockcopy() and blockjob() APIs

So we can properly implement the test virsh_blockcopy.
Also, fix some docstrings of other virsh methods.

Signed-off-by: Yanbing Du <ydu@redhat.com>
Merge pull request #1418 from autotest/virsh-blockcopy

virttest.virsh: Add new blockcopy() and blockjob() APIs