Revert "KVM: x86: zero kvmclock_offset when vcpu0 initializes kvmclock system MSR"

Revert "KVM: x86: zero kvmclock_offset when vcpu0 initializes kvmclock system MSR"

Shifting pvclock_vcpu_time_info.system_time on write to KVM system time
MSR is a change of ABI.  Probably only 2.6.16 based SLES 10 breaks due
to its custom enhancements to kvmclock, but KVM never declared the MSR
only for one-shot initialization.  (Doc says that only one write is
needed.)

This reverts commit 3795633bf017d6a5e4683453686e100aff690b22.
And adds a note to the definition of PVCLOCK_COUNTS_FROM_ZERO.

Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
Acked-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>