pc: limit 64 bit hole to 2G by default

It turns out that some 32 bit windows guests crash
if 64 bit PCI hole size is >2G.
Limit it to 2G for piix and q35 by default.
User may override default 64-bit PCI hole size by
using "pci-hole64-size" property.

Examples:
-global i440FX-pcihost.pci-hole64-size=4G

-global q35-pcihost.pci-hole64-size=4G

Reported-by: Igor Mammedov <imammedo@redhat.com>,
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Message-id: 1375109277-25561-8-git-send-email-imammedo@redhat.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
