Similar to the header split, also the function pointers in BlockDriver
can be split in I/O and global state.

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Message-Id: <20220303151616.325444-26-eesposit@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>