KVM/SVM: Allow direct access to MSR_IA32_SPEC_CTRL

KVM/SVM: Allow direct access to MSR_IA32_SPEC_CTRL

[ Based on a patch from Paolo Bonzini <[email protected]> ]

... basically doing exactly what we do for VMX:

- Passthrough SPEC_CTRL to guests (if enabled in guest CPUID)
- Save and restore SPEC_CTRL around VMExit and VMEntry only if the guest
  actually used it.

Signed-off-by: KarimAllah Ahmed <[email protected]>
Signed-off-by: David Woodhouse <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Reviewed-by: Darren Kenny <[email protected]>
Reviewed-by: Konrad Rzeszutek Wilk <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: Jun Nakajima <[email protected]>
Cc: [email protected]
Cc: Dave Hansen <[email protected]>
Cc: Tim Chen <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Asit Mallick <[email protected]>
Cc: Arjan Van De Ven <[email protected]>
Cc: Greg KH <[email protected]>
Cc: Paolo Bonzini <[email protected]>
Cc: Dan Williams <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Ashok Raj <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]