xen/timer: remove the timer setup loop

With the removal of the usage of the VCPU_SSHOTTMR_future flag, now
all errors from xentimer_vcpu_start_timer should be considered fatal, and
the loop is no longer needed since in case of setting the timer in the past
we will get an event interrupt right away (instead of returning ETIME).

Sponsored by: Citrix Systems R&D
MFC after : 2 weeks