KVM: selftests: Check that the palette table exists before using it

KVM: selftests: Check that the palette table exists before using it

Check that the palette table exists before using it. The maximum number of
AMX palette tables is enumerated by CPUID.1DH:EAX. Assert that the palette
used in amx_test, CPUID.1DH.1H, does not exceed that maximum.

Signed-off-by: Aaron Lewis <aaronlewis@google.com>
Signed-off-by: Mingwei Zhang <mizhang@google.com>
Link: https://lore.kernel.org/r/20230221163655.920289-13-mizhang@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>