vmnetfs: Update cursor chunk/offset/length even at end of I/O

for the convenience of the caller.
vmnetfs: Add chunk_unlock() variant which requires chunk_state lock
vmnetfs: Let transport callers specify a cancel-check callback
vmnetfs: Optionally stream image data in a single request

When we know that we will need the whole image, it's inefficient to
demand-fetch one chunk at a time.  Add config option to fetch the image
in a single large request from a background thread.

Fixes #1.
vmnetfs: Allow returning GError from transport stream callback
vmnetfs: Move check_validators() in transport.c
vmnetfs: Check transport validators before streaming any data
vmnetx.controller.local: Fetch memory images in stream mode