acpi unit-test: verify signature and checksum

Read all ACPI tables from guest - will be useful for further unit tests.

Follow pointers between ACPI tables checking signature and format for
correctness.  Verify checksum for all tables.

Signed-off-by: Marcel Apfelbaum <marcel.a@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
