ivshmem: add 64bit option

ivshmem: add 64bit option

This patch adds a "use64" property which will make the ivshmem driver
register a 64bit memory bar when set, so you have something to play with
when testing 64bit pci bits.  It also allows to have quite big shared
memory regions, like this:

[root@fedora ~]# lspci -vs1:1
01:01.0 RAM memory: Red Hat, Inc Device 1110
        Subsystem: Red Hat, Inc Device 1100
        Physical Slot: 1-1
        Flags: fast devsel
        Memory at fd400000 (32-bit, non-prefetchable) [disabled] [size=256]
        Memory at 8040000000 (64-bit, prefetchable) [size=1G]

[ v5: rebase, update compat property for post-1.2 merge ]
[ v4: rebase & adapt to latest master again ]
[ v3: rebase & adapt to latest master ]
[ v2: default to on as suggested by avi,
      turn off for pc-$old using compat property ]

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Tested-by: Cam Macdonell <cam@cs.ualberta.ca>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>