ACPI: Add vmgenid blob storage to the build tables

This allows them to be centrally initialized and destroyed

The "AcpiBuildTables.vmgenid" array will be used to construct the
"etc/vmgenid_guid" fw_cfg blob.

Its contents will be linked into fw_cfg after being built on the
pc_machine_done() -> acpi_setup() -> acpi_build() call path, and dropped
without use on the subsequent, guest triggered, acpi_build_update() ->
acpi_build() call path.

Signed-off-by: Ben Warren <ben@skyportsystems.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Tested-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
