Fix pc migration from qemu <= 1.5

Fix pc migration from qemu <= 1.5

The following commit introduced a migration incompatibility:

commit 568f0690fd9aa4d39d84b04c1a5dbb53a915c3fe
Author: David Gibson <david@gibson.dropbear.id.au>
Date:   Thu Jun 6 18:48:49 2013 +1000

    pci: Replace pci_find_domain() with more general pci_root_bus_path()

The issue is that i440fx savevm idstr went from 0000:00:00.0/I440FX to
0000:00.0/I440FX. Unfortunately we are stuck with the breakage for
1.6 machine types.

Add a compat property to maintain the busted idstr for the 1.6 machine
types, but revert to the old style format for 1.7+, and <= 1.5.

Tested with migration from qemu 1.5, qemu 1.6, and qemu.git.

Cc: qemu-stable@nongnu.org
Signed-off-by: Cole Robinson <crobinso@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Conflicts:
	include/hw/i386/pc.h