The check cpu->hv_clock.system_time < 0 is redundant since system_time
is a u64 and hence can never be less than zero.  But what was actually
meant is to check that the result is positive, since kernel_ns and
v->kvm->arch.kvmclock_offset are both s64.
Reported-by: NColin King <colin.king@canonical.com>
Suggested-by: NSean Christopherson <sean.j.christopherson@intel.com>
Addresses-Coverity: ("Macro compares unsigned to 0")
Reviewed-by: NMiaohe Lin <linmiaohe@huawei.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>