pc: Disable vmdesc submission for old machines

Older PC machine types might by accident be backwards live migration compatible,
but with the new vmdesc self-describing blob in our live migration stream we
would break that compatibility.

Also users wouldn't expect massive behaviorial differences when updating to a
new version of QEMU while retaining their old machine type, especially not
potential breakage in tooling around live migration.

So disable vmdesc submission for old PC machine types.
Signed-off-by: NAlexander Graf <agraf@suse.de>
Reviewed-by: NDr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: NJuan Quintela <quintela@redhat.com>

hw/i386/pc_piix.c

@@ -335,6 +335,7 @@ static void pc_compat_2_2(MachineState *machine)
                                 CPUID_7_0_EBX_HLE | CPUID_7_0_EBX_RTM, 0);
     x86_cpu_compat_set_features("Broadwell", FEAT_7_0_EBX,
                                 CPUID_7_0_EBX_HLE | CPUID_7_0_EBX_RTM, 0);
+    machine->suppress_vmdesc = true;
 }
 
 static void pc_compat_2_1(MachineState *machine)

hw/i386/pc_q35.c

@@ -314,6 +314,7 @@ static void pc_compat_2_2(MachineState *machine)
                                 CPUID_7_0_EBX_HLE | CPUID_7_0_EBX_RTM, 0);
     x86_cpu_compat_set_features("Broadwell", FEAT_7_0_EBX,
                                 CPUID_7_0_EBX_HLE | CPUID_7_0_EBX_RTM, 0);
+    machine->suppress_vmdesc = true;
 }
 
 static void pc_compat_2_1(MachineState *machine)