The maximum number of struct iovec elements depends on the
BlockDriverState.  The raw-posix and iSCSI protocols have a maximum of
IOV_MAX but others could have different values.

Cc: Peter Lieven <pl@kamp.de>
Suggested-by: NKevin Wolf <kwolf@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>