virtio: move 'use-disabled-flag' property to hw_compat_4_2

Commit 9d7bd082 introduced a new 'use-disabled-flag' property
set to true by default.
To allow the migration, we set this property to false in the hw_compat,
but in the wrong place (hw_compat_4_1).

Since commit 9d7bd082 was released with QEMU 5.0, we move
'use-disabled-flag' property to hw_compat_4_2, so 4.2 machine types
will have the pre-patch behavior and the migration can work.

The issue was discovered with vhost-vsock device and 4.2 machine
type without running any kernel in the VM:
    $ qemu-4.2 -M pc-q35-4.2,accel=kvm \
        -device vhost-vsock-pci,guest-cid=4 \
        -monitor stdio -incoming tcp:0:3333

    $ qemu-5.2 -M pc-q35-4.2,accel=kvm \
        -device vhost-vsock-pci,guest-cid=3 \
        -monitor stdio
    (qemu) migrate -d tcp:0:3333

    # qemu-4.2 output
    qemu-system-x86_64: Failed to load virtio-vhost_vsock:virtio
    qemu-system-x86_64: error while loading state for instance 0x0 of device '0000:00:03.0/virtio-vhost_vsock'
    qemu-system-x86_64: load of migration failed: No such file or directory

Reported-by: Jing Zhao <jinzhao@redhat.com>
Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1907255


Fixes: 9d7bd082 ("virtio-pci: disable vring processing when bus-mastering is disabled")
Cc: mdroth@linux.vnet.ibm.com
CC: qemu-stable@nongnu.org
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Message-Id: <20210108171252.209502-1-sgarzare@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/core/machine.c

@@ -70,12 +70,12 @@ GlobalProperty hw_compat_4_2[] = {
     { "qxl", "revision", "4" },
     { "qxl-vga", "revision", "4" },
     { "fw_cfg", "acpi-mr-restore", "false" },
+    { "virtio-device", "use-disabled-flag", "false" },
 };
 const size_t hw_compat_4_2_len = G_N_ELEMENTS(hw_compat_4_2);
 
 GlobalProperty hw_compat_4_1[] = {
     { "virtio-pci", "x-pcie-flr-init", "off" },
-    { "virtio-device", "use-disabled-flag", "false" },
 };
 const size_t hw_compat_4_1_len = G_N_ELEMENTS(hw_compat_4_1);