pc: limit 64 bit hole to 2G by default

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 <[email protected]>,
Signed-off-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Igor Mammedov <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Reviewed-by: Andreas Färber <[email protected]>
Message-id: 1375109277[email protected]
Signed-off-by: Anthony Liguori <[email protected]>