KVM doesn't fully support Hyper-V reenlightenment notifications on
migration. In particular, it doesn't support emulating TSC frequency
of the source host by trapping all TSC accesses so unless TSC scaling
is supported on the destination host and KVM_SET_TSC_KHZ succeeds, it
is unsafe to proceed with migration.
Normally, we only require KVM_SET_TSC_KHZ to succeed when 'user_tsc_khz'
was set and just 'try' KVM_SET_TSC_KHZ without otherwise.
Introduce a new vmstate section (which is added when the guest has
reenlightenment feature enabled) and add env.tsc_khz to it. We already
have env.tsc_khz packed in 'cpu/tsc_khz' but we don't want to be dependent
on the section order.
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>