We don't have an equivalent to mmap in the qemu block API, so read and
write the bitmap directly.  At least in the dumb implementation added
in this patch this is a lot less efficient, but it means cow can also
work on windows, and over nbd or curl.  And it fixes qemu-iotests testcase
012 which did not work properly due to issues with read-only mmap access.

In addition we can also get rid of the now unused get_mmap_addr function.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>