block: vhdx write support

This adds support for writing to VHDX image files, using coroutines.
Writes into the BAT table goes through the VHDX log.  Currently, BAT
table writes occur when expanding a dynamic VHDX file, and allocating a
new BAT entry.

Signed-off-by: Jeff Cody <jcody@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>