add support for Hyper-V reference time counter

Signed-off: Peter Lieven <pl@kamp.de>
Signed-off: Gleb Natapov
Signed-off: Vadim Rozenfeld <vrozenfe@redhat.com>

After some consideration I decided to submit only Hyper-V reference
counters support this time. I will submit iTSC support as a separate
patch as soon as it is ready.

v1 -> v2
1. mark TSC page dirty as suggested by
    Eric Northup <digitaleric@google.com> and Gleb
2. disable local irq when calling get_kernel_ns,
    as it was done by Peter Lieven <pl@amp.de>
3. move check for TSC page enable from second patch
    to this one.

v3 -> v4
    Get rid of ref counter offset.

v4 -> v5
    replace __copy_to_user with kvm_write_guest
    when updateing iTSC page.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>