xen/events: modify struct evtchn layout

xen/events: modify struct evtchn layout

In order to avoid latent races when updating an event channel put
xen_consumer and pending fields in different bytes. This is no problem
right now, but especially the pending indicator isn't used only when
initializing an event channel (unlike xen_consumer), so any future
addition to this byte would need to be done with a potential race kept
in mind.

At the same time move some other fields around to have less implicit
paddings and to keep related fields more closely together.

Finally switch struct evtchn to no longer use fixed sized types where
not needed.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>