xen/x86: don't lose event interrupts

On slow platforms with unreliable TSC, such as QEMU emulated machines,
it is possible for the FreeBSD kernel to request the next event in the
past. In that case, in the current implementation of
xentimer_vcpu_start_timer, we simply return -ETIME. To be precise Xen
returns -ETIME and we pass it on. As a consequence we need to loop
around to function to make sure that the timer is properly set.

Instead it is better to always ask the hypervisor for a timer event,
even if the timeout is past. To do that, remove the VCPU_SSHOTTMR_future
flag.

Submitted by: Stefano Stabellini <sstabellini@kernel.org>
Reviewed by: royger
MFC after: 2 weeks