acpi: fix OEM ID/OEM Table ID padding

acpi: fix OEM ID/OEM Table ID padding

Commit [2] broke original '\0' padding of OEM ID and OEM Table ID
fields in headers of ACPI tables. While it doesn't have impact on
default values since QEMU uses 6 and 8 characters long values
respectively, it broke usecase where IDs are provided on QEMU CLI.
It shouldn't affect guest (but may cause licensing verification
issues in guest OS).
One of the broken usecases is user supplied SLIC table with IDs
shorter than max possible length, where [2] mangles IDs with extra
spaces in RSDT and FADT tables whereas guest OS expects those to
mirror the respective values of the used SLIC table.

Fix it by replacing whitespace padding with '\0' padding in
accordance with [1] and expectations of guest OS

1) ACPI spec, v2.0b
       17.2 AML Grammar Definition
       ...
       //OEM ID of up to 6 characters. If the OEM ID is
       //shorter than 6 characters, it can be terminated
       //with a NULL character.

2)
Fixes: 602b458201 ("acpi: Permit OEM ID and OEM table ID fields to be changed")
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/707
Reported-by: Dmitry V. Orekhov <dima.orekhov@gmail.com>
Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Cc: qemu-stable@nongnu.org
Message-Id: <20220112130332.1648664-4-imammedo@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Ani Sinha <ani@anisinha.ca>
Tested-by: Dmitry V. Orekhov dima.orekhov@gmail.com