pc: add 'etc/reserved-memory-end' fw_cfg interface for SeaBIOS
'etc/reserved-memory-end' will allow QEMU to tell BIOS where PCI
BARs mapping could safely start in high memory.
Allowing BIOS to start mapping 64-bit PCI BARs at address where it
wouldn't conflict with other mappings QEMU might place before it.
That permits QEMU to reserve extra address space before
64-bit PCI hole for memory hotplug.
Signed-off-by: Igor Mammedov <imammedo@redhat.com> Acked-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com> Reviewed-by: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>