KVM: x86: Get the number of Hyper-V sparse banks from the VARHEAD field

Get the number of sparse banks from the VARHEAD field, which the guest is
required to provide as "The size of a variable header, in QWORDS.", where
the variable header is:

  Variable Header Bytes = {Total Header Bytes - sizeof(Fixed Header)}
                          rounded up to nearest multiple of 8
  Variable HeaderSize = Variable Header Bytes / 8

In other words, the VARHEAD should match the number of sparse banks.
Keep the manual count as a sanity check, but otherwise rely on the field
so as to more closely align with the logic defined in the TLFS and to
allow for future cleanups.

Tweak the tracepoint output to use "rep_cnt" instead of simply "cnt" now
that there is also "var_cnt".

Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20211207220926.718794-3-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>