Change the bdev init signatures

This patch changes all the bdev.BlockDev constructors to take an
additional ‘size’ parameter, all the backend functions that call those
functions to pass it and also changes backend.BlocdevCreate() to not use
the size passed via the rpc call but instead directly disk.size (this is
the only way it's called).

Note that this patch doesn't do anything with this parameter, just
stores it on the blockdev objects.

With the patch, we actually have a more uniform init sequence (before
create had the parameter, but the other functions not).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>