lock down hypercall continuation encoding masks

Andrew validly points out that even if these masks aren't a formal part
of the hypercall interface, we aren't free to change them: A guest
suspended for migration in the middle of a continuation would fail to
work if resumed on a hypervisor using a different value. Hence add
respective comments to their definitions.

Additionally, to help future extensibility as well as in the spirit of
reducing undefined behavior as much as possible, refuse hypercalls made
with the respective bits non-zero when the respective sub-ops don't
make use of those bits.

Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Tim Deegan <tim@xen.org>
Release-Acked-by: Konrad Rzeszutek Wilk <Konrad.wilk@oracle.com>
have architectures specify the number of PIRQs a hardware domain gets

The current value of nr_static_irqs + 256 is often too small for larger
systems. Make it dependent on CPU count and number of IO-APIC pins on
x86, and (until it obtains PCI support) simply NR_IRQS on ARM.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: David Vrabel <david.vrabel@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Release-Acked-by: Konrad Rzeszutek Wilk <Konrad.wilk@oracle.com>