KVM: nVMX: WARN if subtly-impossible VMFUNC conditions occur

WARN and inject #UD when emulating VMFUNC for L2 if the function is
out-of-bounds or if VMFUNC is not enabled in vmcs12.  Neither condition
should occur in practice, as the CPU is supposed to prioritize the #UD
over VM-Exit for out-of-bounds input and KVM is supposed to enable
VMFUNC in vmcs02 if and only if it's enabled in vmcs12, but neither of
those dependencies is obvious.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210609234235.1244004-15-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>