xen: remove evtchn_upcall_mask from interface on ARM

xen: remove evtchn_upcall_mask from interface on ARM

On ARM event-channel upcalls are masked using the hardware's interrupt mask
bit and not by a software bit.

Leaving this field present in the interface has caused some confusion already
and is liable to mean it gets inadvertently used in the future. So arrange for
this field to be turned into a padding field on ARM by introducing a
XEN_HAVE_PV_UPCALL_MASK define.

This bit is also unused for x86 PV-on-HVM guests, but we can't realistically
distinguish those from x86 PV guests in the headers.

Add a per-arch vcpu_event_delivery_is_enabled function to replace an open
coded use of evtchn_upcall_mask in common code (in a debug keyhandler).  The
existing local_event_delivery_is_enabled, which operates only on current, was
unimplemented on ARM and unused on x86, so remove it.

ifdef the use of evtchn_upcall_mask when setting up a new vcpu info page.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Keir Fraser <keir@xen.org>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>