Check 'td_owepreempt' and yield the vcpu thread if it is set.

This is done explicitly because a vcpu thread can be in a critical section
for the entire time slice alloted to it. This in turn can delay the handling
of the 'td_owepreempt'.

Reviewed by: jhb
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D2430