KVM: x86: use CPUID to locate host page table reserved bits

KVM: x86: use CPUID to locate host page table reserved bits

[ Upstream commit 7adacf5eb2d2048045d9fd8fdab861fd9e7e2e96 ]

The comment in kvm_get_shadow_phys_bits refers to MKTME, but the same is actually
true of SME and SEV.  Just use CPUID[0x8000_0008].EAX[7:0] unconditionally if
available, it is simplest and works even if memory is not encrypted.

Cc: stable@vger.kernel.org
Reported-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>