qemu: make Hyperv settings exclusive to x86 and aarch64

Hyperv features are supported by both x86 and aarch64. The <hyperv/>
declaration in the XML by itself is benign to other architectures,
but any of its 14 current features will break QEMU with an error
like this (from ppc64):

qemu-kvm: Expected key=value format, found hv_relaxed

This is a more extreme case than the one for apic eoi because we
would need an extra 'switch' statement, with all current Hyperv
features in the body of qemuDomainDefValidateFeatures(), to
check if the user attempted to activate any of them. It's easier to
simply fail to launch with any 'hyperv' declaration in the XML for
every arch which is not x86 and aarch64.

A fair disclaimer about Windows and PowerPC: the last Windows version
that ran in the architecture is the hall of famer Windows NT 4.0,
launched in 1996 and with end of extended support for the Server
version in 2004 [1]. I am acknowledging that there might be Windows
NT 4.0 users running in PowerPC, but not enough people running it
under KVM/QEMU to justify Libvirt allowing 'hyperv' to exist in the
domain XML of ppc64 domains.

[1] https://en.wikipedia.org/wiki/Windows_NT_4.0Signed-off-by: NDaniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: NAndrea Bolognani <abologna@redhat.com>

src/qemu/qemu_domain.c

@@ -5284,12 +5284,23 @@ qemuDomainDefValidateFeatures(const virDomainDef *def,
             }
             break;
 
+        case VIR_DOMAIN_FEATURE_HYPERV:
+            if (def->features[i] != VIR_TRISTATE_SWITCH_ABSENT &&
+                !ARCH_IS_X86(def->os.arch) && !qemuDomainIsARMVirt(def)) {
+                virReportError(VIR_ERR_CONFIG_UNSUPPORTED,
+                               _("Hyperv features are not supported for "
+                                 "architecture '%s' or machine type '%s'"),
+                                 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:
         case VIR_DOMAIN_FEATURE_PRIVNET:
-        case VIR_DOMAIN_FEATURE_HYPERV:
         case VIR_DOMAIN_FEATURE_CAPABILITIES:
         case VIR_DOMAIN_FEATURE_PMU:
         case VIR_DOMAIN_FEATURE_MSRS: