ArmVirtPkg/PlatformHasAcpiDtDxe: allow guest level ACPI disable override

ArmVirtPkg/PlatformHasAcpiDtDxe: allow guest level ACPI disable override

In general, we should not present two separate (and inevitably different)
hardware descriptions to the OS, in the form of ACPI tables and a device
tree blob. For this reason, we recently added the logic to ArmVirtQemu to
only expose the ACPI 2.0 entry point if no DT binary is being passed, and
vice versa.

However, this is arguably a regression for those who relied on DT
descriptions being available, even if the former behavior can be
restored by passing the -no-acpi switch to QEMU.

So allow a secret handshake with the UEFI Shell, to set a variable that
will result in ACPI to be disabled on subsequent boots even if -no-acpi
was not passed on the QEMU command line.

  setvar -nv -bs -guid 50bea1e5-a2c5-46e9-9b3a-59596516b00a ForceNoAcpi =01

To delete the variable and revert to the old situation, simply omit the
value after the =

  setvar -nv -bs -guid 50bea1e5-a2c5-46e9-9b3a-59596516b00a ForceNoAcpi =

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Tested-by: Marc Zyngier <marc.zyngier@arm.com>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>