cmdline: "extra_guest_irqs" is inapplicable to PVH

PVH in particular has no (externally visible) notion of pIRQ-s. Mention
that in the description of the respective command line option and have
arch_hwdom_irqs() also reflect this (thus suppressing the log message
there as well, as being pretty meaningless in this case anyway).

Suggested-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>
Release-Acked-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>

docs/misc/xen-command-line.pandoc

@@ -1178,7 +1178,8 @@ versa.  For example to change dom0 without changing domU, use
 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.
+to use the default.  Note further that the Dom0 setting has no useful meaning
+for the PVH case; use of the option may have an adverse effect there, though.
 
 ### ext_regions (Arm)
 > `= <boolean>`

xen/arch/x86/io_apic.c

@@ -2670,6 +2670,10 @@ unsigned int __hwdom_init arch_hwdom_irqs(const struct domain *d)
     if ( is_system_domain(d) )
         return max_irqs;
 
+    /* PVH (generally: HVM) can't use PHYSDEVOP_pirq_eoi_gmfn_v{1,2}. */
+    if ( is_hvm_domain(d) )
+        return nr_irqs;
+
     if ( !d->domain_id )
         n = min(n, dom0_max_vcpus());
     n = min(nr_irqs_gsi + n * NR_DYNAMIC_VECTORS, max_irqs);