block: Reorganize some declarations in block-backend-io.h

block: Reorganize some declarations in block-backend-io.h

Keep generated_co_wrapper and coroutine_fn pairs together. This should
make it clear that each I/O function has these two versions.

Also move blk_co_{pread,pwrite}()'s implementations out of the header
file for consistency.

Signed-off-by: Alberto Faria <afaria@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20220705161527.1054072-18-afaria@redhat.com>
Reviewed-by: Hanna Reitz <hreitz@redhat.com>
Signed-off-by: Hanna Reitz <hreitz@redhat.com>