cpu_riscv64.c: add update() implementation

At this moment it is not possible to launch a 'riscv64' domain if a CPU
definition is presented in the domain. For example, adding this CPU
definition:

  <cpu mode='custom' match='exact' check='none'>
    <model fallback='forbid'>rv64</model>
  </cpu>

Will trigger the following error:

$ sudo ./run tools/virsh start riscv-virt1
error: Failed to start domain 'riscv-virt1'
error: this function is not supported by the connection driver:
       cannot update guest CPU for riscv64 architecture

The error comes from virCPUUpdate(), via qemuProcessUpdateGuestCPU(),
and it's caused by the absence of the 'update' API in the existing
RISC-V driver.

Add an 'update' API impl to the RISC-V driver to allow for CPU
definitions to be declared in RISC-V domains. This API was copied from
the ARM driver (virCPUarmUpdate()) since it's a good enough
implementation to get us going.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>

po/POTFILES

@@ -70,6 +70,7 @@ src/cpu/cpu.c
 src/cpu/cpu_arm.c
 src/cpu/cpu_map.c
 src/cpu/cpu_ppc64.c
+src/cpu/cpu_riscv64.c
 src/cpu/cpu_s390.c
 src/cpu/cpu_x86.c
 src/datatypes.c

src/cpu/cpu_riscv64.c

@@ -46,6 +46,32 @@ virCPURiscv64ValidateFeatures(virCPUDef *cpu G_GNUC_UNUSED)
 }
 
 
+static int
+virCPURiscv64Update(virCPUDef *guest,
+                    const virCPUDef *host,
+                    bool relative)
+{
+    g_autoptr(virCPUDef) updated = virCPUDefCopyWithoutModel(guest);
+
+    if (!relative || guest->mode != VIR_CPU_MODE_HOST_MODEL)
+        return 0;
+
+    if (!host) {
+        virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
+                       _("unknown host CPU model"));
+        return -1;
+    }
+
+    updated->mode = VIR_CPU_MODE_CUSTOM;
+    virCPUDefCopyModel(updated, host, true);
+
+    virCPUDefStealModel(guest, updated, false);
+    guest->mode = VIR_CPU_MODE_CUSTOM;
+    guest->match = VIR_CPU_MATCH_EXACT;
+
+    return 0;
+}
+
 struct cpuArchDriver cpuDriverRiscv64 = {
     .name = "riscv64",
     .arch = archs,
@@ -54,6 +80,6 @@ struct cpuArchDriver cpuDriverRiscv64 = {
     .decode     = NULL,
     .encode     = NULL,
     .baseline   = NULL,
-    .update     = NULL,
+    .update     = virCPURiscv64Update,
     .validateFeatures = virCPURiscv64ValidateFeatures,
 };