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 boundaries by using
"pci_hole64_end " property.

Examples:
-global i440FX-pcihost.pci_hole64_end=0xffffffffffffffff

-global q35-pcihost.pci_hole64_end=0xffffffffffffffff

Reported-by: Igor Mammedov <imammedo@redhat.com>,
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Igor Mammedov <imammedo@redhat.com>