hw/arm/virt: Fix property "gic-version" error handling

virt_set_gic_version() calls exit(1) when passed an invalid property
value.  Property setters are not supposed to do that.  Screwed up in
commit b92ad394.  Harmless, because the property belongs to a machine.
Set an error object instead.

Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: qemu-arm@nongnu.org
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>

hw/arm/virt.c

@@ -1200,9 +1200,8 @@ static void virt_set_gic_version(Object *obj, const char *value, Error **errp)
     } else if (!strcmp(value, "host")) {
         vms->gic_version = 0; /* Will probe later */
     } else {
-        error_report("Invalid gic-version option value");
-        error_printf("Allowed gic-version values are: 3, 2, host\n");
-        exit(1);
+        error_setg(errp, "Invalid gic-version value");
+        error_append_hint(errp, "Valid values are 3, 2, host.\n");
     }
 }