cmdline: document and enforce "extra_guest_irqs" upper bounds

PHYSDEVOP_pirq_eoi_gmfn_v<N> accepting just a single GFN implies that no
more than 32k pIRQ-s can be used by a domain on x86. Document this upper
bound.

To also enforce the limit, (ab)use both arch_hwdom_irqs() (changing its
parameter type) and setup_system_domains(). This is primarily to avoid
exposing the two static variables or introducing yet further arch hooks.

While touching arch_hwdom_irqs() also mark it hwdom-init.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>

amend 'cmdline: document and enforce "extra_guest_irqs" upper bounds'

Address late review comments for what is now commit 17f6d398f7:
- bound max_irqs right away against nr_irqs
- introduce a #define for a constant used twice

Requested-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
master commit: 17f6d398f7
master date: 2024-07-02 12:00:27 +0200
master commit: 1f56accba3
master date: 2024-07-03 14:03:27 +0200

docs/misc/xen-command-line.pandoc

@@ -1141,7 +1141,8 @@ common for all domUs, while the optional second number (preceded by a comma)
 is for dom0.  Changing the setting for domU has no impact on dom0 and vice
 versa.  For example to change dom0 without changing domU, use
 `extra_guest_irqs=,512`.  The default value for Dom0 and an eventual separate
-hardware domain is architecture dependent.
+hardware domain is architecture dependent.  The upper limit for both values on
+x86 is such that the resulting total number of IRQs can't be higher than 32768.
 Note that specifying zero as domU value means zero, while for dom0 it means
 to use the default.