KVM: X86: Add functions for retrieving L2 TSC fields from common code

In order to implement as much of the nested TSC scaling logic as
possible in common code, we need these vendor callbacks for retrieving
the TSC offset and the TSC multiplier that L1 has set for L2.

Signed-off-by: Ilias Stamatis <ilstam@amazon.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Message-Id: <20210526184418.28881-7-ilstam@amazon.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>