virsh: Refactor block job waiting in cmdBlockPull

virsh: Refactor block job waiting in cmdBlockPull

Introduce helper function that will provide logic for waiting for block
job completion so the 3 open coded places can be unified and improved.

This patch introduces the whole logic and uses it to fix
cmdBlockJobPull. The vshBlockJobWait function provides common logic for
block job waiting that should be robust enough to work across all
previous versions of libvirt. Since virsh allows passing user-provided
strings as paths of block devices we can't reliably use block job events
for detection of block job states so the function contains a great deal
of fallback logic.