tests: acpi: use AcpiSdtTable::aml instead of AcpiSdtTable::header::signature

tests: acpi: use AcpiSdtTable::aml instead of AcpiSdtTable::header::signature

AcpiSdtTable::header::signature is the only remained field from
AcpiTableHeader structure used by tests. Instead of using packed
structure to access signature, just read it directly from table
blob and remove no longer used AcpiSdtTable::header / union and
keep only AcpiSdtTable::aml byte array.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Acked-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>