VMX: Rework and enhance initial feature detection/enabling

So far we ran into an unhandled GP when VMX was unavailable. Change the
logic to handle this gracefully. Rework test_vmx_capability to
test_vmx_feature_control which not only enables VMX in the feature
control MSR but also test related error behavior.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>