x86/spectre_v2: Don't check microcode versions when running under hypervisors

As:

 1) It's known that hypervisors lie about the environment anyhow (host
    mismatch)
 
 2) Even if the hypervisor (Xen, KVM, VMWare, etc) provided a valid
    "correct" value, it all gets to be very murky when migration happens
    (do you provide the "new" microcode of the machine?).

And in reality the cloud vendors are the ones that should make sure that
the microcode that is running is correct and we should just sing lalalala
and trust them.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Cc: Wanpeng Li <kernellwp@gmail.com>
Cc: kvm <kvm@vger.kernel.org>
Cc: Krčmář <rkrcmar@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
CC: "H. Peter Anvin" <hpa@zytor.com>
CC: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/20180226213019.GE9497@char.us.oracle.com