powerpc: fix -machine usb=no for newworld and pseries machines

Capture the explicit setting of "usb=no" into a separate bool, and
use it to skip the update of machine->usb in the board init function.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NAlexander Graf <agraf@suse.de>

hw/core/machine.c

@@ -223,6 +223,7 @@ static void machine_set_usb(Object *obj, bool value, Error **errp)
     MachineState *ms = MACHINE(obj);
 
     ms->usb = value;
+    ms->usb_disabled = !value;
 }
 
 static char *machine_get_firmware(Object *obj, Error **errp)

hw/ppc/mac_newworld.c

@@ -371,7 +371,7 @@ static void ppc_core99_init(MachineState *machine)
         /* 970 gets a U3 bus */
         pci_bus = pci_pmac_u3_init(pic, get_system_memory(), get_system_io());
         machine_arch = ARCH_MAC99_U3;
-        machine->usb |= defaults_enabled();
+        machine->usb |= defaults_enabled() && !machine->usb_disabled;
     } else {
         pci_bus = pci_pmac_init(pic, get_system_memory(), get_system_io());
         machine_arch = ARCH_MAC99;

hw/ppc/spapr.c

@@ -1580,7 +1580,7 @@ static void ppc_spapr_init(MachineState *machine)
     /* Graphics */
     if (spapr_vga_init(phb->bus)) {
         spapr->has_graphics = true;
-        machine->usb |= defaults_enabled();
+        machine->usb |= defaults_enabled() && !machine->usb_disabled;
     }
 
     if (machine->usb) {

include/hw/boards.h

@@ -146,6 +146,7 @@ struct MachineState {
     bool dump_guest_core;
     bool mem_merge;
     bool usb;
+    bool usb_disabled;
     char *firmware;
     bool iommu;
     bool suppress_vmdesc;