qemu: avoid launching non-x86 guests with APIC-EOI setting

The "<apic/>" feature, although it's available only for x86 guests,
can be declared in the domain XML of other archs without errors.
But setting its 'eoi' attribute will break QEMU. For "<apic eoi='on'/>",
in a ppc64 guest:

qemu-kvm: Expected key=value format, found +kvm_pv_eoi

A similar error happens with eoi='off'.

One can argue that it's better to simply forbid launching non-x86
guests with "<apic/>" declared in the XML - it is a feature that
the architecture doesn't support and this would make it clearer
about it. This is sensible, but there are non-x86 guests that are
running with "<apic/>" declared in the domain (and A LOT of guests
running with "<acpi/>" for that matter, probably reminiscent of x86
templates that were reused for other archs) that will stop working if we
go this route.

A more subtle approach is to detect if the 'eoi' element is being set
for non-x86 guests and warn the user about it with a better error
message than the one QEMU provides. This is the new error message
when any value is set for the 'eoi' element in a ppc64 XML:

error: unsupported configuration: The 'eoi' attribute of the 'apic'
feature is not supported for architecture 'ppc64' or machine type
'pseries'.

https://bugzilla.redhat.com/show_bug.cgi?id=1236440Signed-off-by: NDaniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: NAndrea Bolognani <abologna@redhat.com>

src/qemu/qemu_domain.c

@@ -5255,8 +5255,23 @@ qemuDomainDefValidateFeatures(const virDomainDef *def,
             }
             break;
 
-        case VIR_DOMAIN_FEATURE_ACPI:
         case VIR_DOMAIN_FEATURE_APIC:
+            /* The kvm_pv_eoi feature is x86-only. */
+            if (def->features[i] != VIR_TRISTATE_SWITCH_ABSENT &&
+                def->apic_eoi != VIR_TRISTATE_SWITCH_ABSENT &&
+                !ARCH_IS_X86(def->os.arch)) {
+                virReportError(VIR_ERR_CONFIG_UNSUPPORTED,
+                               _("The 'eoi' attribute of the '%s' feature "
+                                 "is not supported for architecture '%s' or "
+                                 "machine type '%s'"),
+                                 featureName,
+                                 virArchToString(def->os.arch),
+                                 def->os.machine);
+                 return -1;
+            }
+            break;
+
+        case VIR_DOMAIN_FEATURE_ACPI:
         case VIR_DOMAIN_FEATURE_PAE:
         case VIR_DOMAIN_FEATURE_HAP:
         case VIR_DOMAIN_FEATURE_VIRIDIAN: