x86/xen: eliminate some private header files

Under arch/x86/xen there is one large private header file xen-ops.h
containing most of the Xen-private x86 related declarations, and then
there are several small headers with a handful of declarations each.

Merge the small headers into xen-ops.h.

While doing that, move the declaration of xen_fifo_events from
xen-ops.h into include/xen/events.h where it should have been from the
beginning.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Message-ID: <20240710093718.14552-3-jgross@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>