bios-linker-loader: document+validate input

While guest/host ABI is documented in hw/acpi/bios-linker-loader.c,
the API was left undocumented.

This adds documentation for all API functions.

Additionally, input is validated to make sure all
pointers fall within range of provided files.

To allow this validation for checksum commands,
bios_linker_loader_add_checksum is changed to accept GArray * in place
of void *.

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