KVM: nVMX: Change emulated VMX-preemption timer hrtimer to absolute

Prepare for migration of this hrtimer, by changing it from relative to
absolute. (I couldn't get migration to work with a relative timer.)

Signed-off-by: Jim Mattson <jmattson@google.com>
Reviewed-by: Peter Shier <pshier@google.com>
Reviewed-by: Oliver Upton <oupton@google.com>
Message-Id: <20200508203643.85477-3-jmattson@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>