KVM: selftests: Allow kvm_cpu_has_ept() to be called on AMD CPUs

KVM: selftests: Allow kvm_cpu_has_ept() to be called on AMD CPUs

In preparation for generalizing the nested dirty logging test, checking
if either EPT or NPT is enabled will be needed. To avoid needing to gate
the kvm_cpu_has_ept() call by the CPU type, make sure the function
returns false if VMX is not available instead of trying to read VMX-only
MSRs.

No functional change intended.

Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev>
Link: https://patch.msgid.link/20251230230150.4150236-16-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>