xio: prepare for splitting XioMsg - methods will access members that should go to...

xio: prepare for splitting XioMsg - methods will access members that should go to base class

data members that are intended to be in the base class should be accessed
only thru a method (next, the method will be virtual in the base class)

Note: XioMsg is going to be split into two classes:
1. a base class for any xio_msg message
2. a derived class for xio_msg objects that contains Ceph messages
(than will be able to use the base class for command messages such as
ack, keepallive that xio doesn't support at the moment)

Signed-off-by: Avner BenHanoch <avnerb@mellanox.com>