ArmVirtPkg: restrict mapping attributes of normal memory to EFI_MEMORY_WB

ArmVirtPkg: restrict mapping attributes of normal memory to EFI_MEMORY_WB

In general, on an ARM system, mapping normal memory as device memory may
have unintended side effects, given that unaligned accesses or loads and
stores with special semantics (e.g., load/store exclusive) may fault or
may not work as expected.

Under KVM, the situation is even worse, since the host may not expect the
guest to perform uncached accesses, and so writes to such an uncached
region may get lost completely.

Since the only safe mapping type under KVM is EFI_MEMORY_WB, remove all
other memory type attributes.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>