Move lib/bdev.py to lib/block/bdev.py

That's in preparation of extracting DRBD related code from bdev.py. As
bdev.py is already rather long, new features will require to split it
into more manageable pieces. That's why it's moved in an own directory.

Signed-off-by: Thomas Thrainer <thomasth@google.com>
Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>