map 64-bit PCI BARs at location provided by emulator

Currently 64-bit PCI BARs are unconditionally mapped by BIOS right
over 4G + RamSizeOver4G location, which doesn't allow to reserve
extra space before 64-bit PCI window. For memory hotplug an extra
RAM space might be reserved after present 64-bit RAM end and BIOS
should map 64-bit PCI BARs after it.

Introduce "etc/reserved-memory-end" romfile to provide BIOS a hint
where it should start mapping of 64-bit PCI BARs. If romfile is
missing, BIOS reverts to legacy behavior and starts mapping after
high memory.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
---
v3:
  * s/pcimem64-start/reserved-memory-end/
v2:
  * place 64-bit window behind high RAM end if "etc/pcimem64-start"
    point below it.