i386/kvm: set tsc_khz before configuring Hyper-V CPUID

Timing-related Hyper-V enlightenments will benefit from knowing the final
tsc_khz value. This commit just moves the code in preparation for further
changes.

Signed-off-by: Ladi Prosek <lprosek@redhat.com>
Message-Id: <20170807085703.32267-3-lprosek@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>