pc: add compat_props placeholder for 2.0 machine type

Add the "boilerplate" necessary for subsequent patches to
simply drop in compat_props for pc machines 2.0 and older.

This patch contains no functional changes.
Signed-off-by: NGabriel Somlo <somlo@cmu.edu>
Acked-by: NAlexander Graf <agraf@suse.de>
Reviewed-by: NAndreas Färber <afaerber@suse.de>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

hw/i386/pc_piix.c

@@ -414,6 +414,10 @@ static QEMUMachine pc_i440fx_machine_v2_0 = {
     PC_I440FX_2_0_MACHINE_OPTIONS,
     .name = "pc-i440fx-2.0",
     .init = pc_init_pci_2_0,
+    .compat_props = (GlobalProperty[]) {
+        PC_COMPAT_2_0,
+        { /* end of list */ }
+    },
 };
 
 #define PC_I440FX_1_7_MACHINE_OPTIONS PC_I440FX_MACHINE_OPTIONS

hw/i386/pc_q35.c

@@ -327,6 +327,10 @@ static QEMUMachine pc_q35_machine_v2_0 = {
     PC_Q35_2_0_MACHINE_OPTIONS,
     .name = "pc-q35-2.0",
     .init = pc_q35_init_2_0,
+    .compat_props = (GlobalProperty[]) {
+        PC_Q35_COMPAT_2_0,
+        { /* end of list */ }
+    },
 };
 
 #define PC_Q35_1_7_MACHINE_OPTIONS PC_Q35_MACHINE_OPTIONS

include/hw/i386/pc.h

@@ -242,8 +242,12 @@ int e820_add_entry(uint64_t, uint64_t, uint32_t);
 int e820_get_num_entries(void);
 bool e820_get_entry(int, uint32_t, uint64_t *, uint64_t *);
 
+#define PC_Q35_COMPAT_2_0 \
+        PC_COMPAT_2_0
+
 #define PC_Q35_COMPAT_1_7 \
         PC_COMPAT_1_7, \
+        PC_Q35_COMPAT_2_0, \
         {\
             .driver   = "hpet",\
             .property = HPET_INTCAP,\
@@ -262,7 +266,12 @@ bool e820_get_entry(int, uint32_t, uint64_t *, uint64_t *);
         PC_COMPAT_1_4, \
         PC_Q35_COMPAT_1_5
 
+#define PC_COMPAT_2_0 \
+        {\
+        }
+
 #define PC_COMPAT_1_7 \
+        PC_COMPAT_2_0, \
         {\
             .driver   = TYPE_USB_DEVICE,\
             .property = "msos-desc",\