fw_cfg: Migrate ACPI table mr sizes separately

Any sub-page size update to ACPI MRs will be lost during
migration, as we use aligned size in ram_load_precopy() ->
qemu_ram_resize() path. This will result in inconsistency in
FWCfgEntry sizes between source and destination. In order to avoid
this, save and restore them separately during migration.

Up until now, this problem may not be that relevant for x86 as both
ACPI table and Linker MRs gets padded and aligned. Also at present,
qemu_ram_resize() doesn't invoke callback to update FWCfgEntry for
unaligned size changes. But since we are going to fix the
qemu_ram_resize() in the subsequent patch, the issue may become
more serious especially for RSDP MR case.

Moreover, the issue will soon become prominent in arm/virt as well
where the MRs are not padded or aligned at all and eventually have
acpi table changes as part of future additions like NVDIMM hot-add
feature.

Suggested-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Shameer Kolothum <shameerali.kolothum.thodi@huawei.com>
Acked-by: David Hildenbrand <david@redhat.com>
Message-Id: <20200403101827.30664-3-shameerali.kolothum.thodi@huawei.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/core/machine.c

@@ -39,6 +39,7 @@ GlobalProperty hw_compat_4_2[] = {
     { "usb-redir", "suppress-remote-wake", "off" },
     { "qxl", "revision", "4" },
     { "qxl-vga", "revision", "4" },
+    { "fw_cfg", "acpi-mr-restore", "false" },
 };
 const size_t hw_compat_4_2_len = G_N_ELEMENTS(hw_compat_4_2);