Determining the size of a field is useful when you don't have a struct
variable handy.  Open-coding this is ugly.

This patch adds the sizeof_field() macro, which is similar to
typeof_field().  Existing instances are updated to use the macro.
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NJohn Snow <jsnow@redhat.com>
Message-id: 20180614164431.29305-1-stefanha@redhat.com
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>