pc: add 'etc/reserved-memory-end' fw_cfg interface for SeaBIOS

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 <[email protected]>
Acked-by: Peter Crosthwaite <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>