qemu: blockjob: Store list of bitmaps disabled prior to commit

qemu: blockjob: Store list of bitmaps disabled prior to commit

Starting a commit job will require disabling bitmaps in the base image
so that they are not dirtied by the commit job. We need to store a list
of the bitmaps so that we can later re-enable them.

Add a field and status XML handling code as well as a test.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>