qemuProcessRefreshCPU: skip 'host-model' logic for pSeries guests

Commit v3.10.0-182-g237f045d ("qemu: Ignore fallback CPU attribute
on reconnect") forced CPU 'fallback' to ALLOW, regardless of user
choice. This fixed a situation in which guests created with older
Libvirt versions, which used CPU mode 'host-model' in runtime, would
fail to launch in a newer Libvirt if the fallback was set to FORBID.
This would lead to a scenario where the CPU was translated to 'host-model'
to 'custom', but then the FORBID setting would make the translation
process fail.

PSeries can operate with 'host-model' in runtime due to specific PPC64
mechanics regarding compatibility mode. The update() implementation of
the cpuDriverPPC64 driver is a NO-OP if CPU mode is 'host-model', and
the driver does not implement translate(). The commit mentioned above
is causing PSeries guests to get their 'fallback' setting to ALLOW,
overwriting user choice, exposing a design problem in
qemuProcessRefreshCPU() - for PSeries guests, handling 'host-model'
as it is being done does not apply.

All other cpuArchDrivers implements update() and changes guest mode
to VIR_CPU_MODE_CUSTOM, meaning that PSeries is currently the only
exception to this logic. Let's make it official.

https://bugzilla.redhat.com/show_bug.cgi?id=1660711Suggested-by: NJiri Denemark <jdenemar@redhat.com>
Signed-off-by: NDaniel Henrique Barboza <danielhb413@gmail.com>
Message-Id: <20200525123945.4049591-2-danielhb413@gmail.com>
Reviewed-by: NJiri Denemark <jdenemar@redhat.com>

src/qemu/qemu_process.c

@@ -7775,6 +7775,15 @@ qemuProcessRefreshCPU(virQEMUDriverPtr driver,
      * running domain.
      */
     if (vm->def->cpu->mode == VIR_CPU_MODE_HOST_MODEL) {
+        /*
+         * PSeries domains are able to run with host-model CPU by design,
+         * even on Libvirt newer than 2.3, never replacing host-model with
+         * custom in the virCPUUpdate() call. It is not needed to call
+         * virCPUUpdate() and qemuProcessUpdateCPU() in this case.
+         */
+        if (qemuDomainIsPSeries(vm->def))
+            return 0;
+
         if (!(hostmig = virCPUCopyMigratable(host->arch, host)))
             return -1;