cleanup bios_linker_loader_add_pointer() API by switching
arguments to taking offsets relative to corresponding files
instead of doing pointer arithmetic on behalf of user which
were confusing.

Also make offset inside of source file explicit in API
so that user won't have to manually set it in
destination file blob and while at it add additional
boundary checks.
Signed-off-by: NIgor Mammedov <imammedo@redhat.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>