KVM: SVM: Delay restoration of host MSR_TSC_AUX until return to userspace

Use KVM's "user return MSRs" framework to defer restoring the host's
MSR_TSC_AUX until the CPU returns to userspace.  Add/improve comments to
clarify why MSR_TSC_AUX is intercepted on both RDMSR and WRMSR, and why
it's safe for KVM to keep the guest's value loaded even if KVM is
scheduled out.

Cc: Reiji Watanabe <reijiw@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210423223404.3860547-5-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>