Similarly to the previous patches, split block-backend.h
in block-backend-io.h and block-backend-global-state.h

In addition, remove "block/block.h" include as it seems
it is not necessary anymore, together with "qemu/iov.h"

block-backend-common.h contains the structures shared between
the two headers, and the functions that can't be categorized as
I/O or global state.

Assertions are added in the next patch.

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Message-Id: <20220303151616.325444-8-eesposit@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>