migration: allow machine to enforce configuration section migration

Migration of pseries-2.3 doesn't have configuration section. Unfortunately,
QEMU 2.4/2.4.1/2.5 are buggy and always stream and expect the configuration
section, and break migration both ways.

This patch introduces a property which allows to enforce a configuration
section for machines who don't have one.

It can be set at startup:

-machine enforce-config-section=on

or later from the QEMU monitor:

qom-set /machine enforce-config-section on

It is up to the tooling to set or unset this property according to the
version of the QEMU at the other end of the pipe.

Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

include/hw/boards.h

@@ -127,6 +127,7 @@ struct MachineState {
     char *firmware;
     bool iommu;
     bool suppress_vmdesc;
+    bool enforce_config_section;
 
     ram_addr_t ram_size;
     ram_addr_t maxram_size;