KVM: SVM: fix tsc scaling cache logic

SVM uses a per-cpu variable to cache the current value of the
tsc scaling multiplier msr on each cpu.

Commit 1ab9287a
("KVM: X86: Add vendor callbacks for writing the TSC multiplier")
broke this caching logic.

Refactor the code so that all TSC scaling multiplier writes go through
a single function which checks and updates the cache.

This fixes the following scenario:

1. A CPU runs a guest with some tsc scaling ratio.

2. New guest with different tsc scaling ratio starts on this CPU
   and terminates almost immediately.

   This ensures that the short running guest had set the tsc scaling ratio just
   once when it was set via KVM_SET_TSC_KHZ. Due to the bug,
   the per-cpu cache is not updated.

3. The original guest continues to run, it doesn't restore the msr
   value back to its own value, because the cache matches,
   and thus continues to run with a wrong tsc scaling ratio.

Fixes: 1ab9287a

 ("KVM: X86: Add vendor callbacks for writing the TSC multiplier")
Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Message-Id: <20220606181149.103072-1-mlevitsk@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>