Only block if when waiting for a non-elapsed time.

A long long time ago on a machine far far away (2 desks away!) there
existed... the dual core machine!  These machines would often share a
single cpu among guest VMs.  When one VM gets spun up sith lords block
other VMs for possibly hundreds of milliseconds.  In this elapsed time a
computed "time to block" might have passed, leaving it frozen in
carbonite until 2^64 nanoseconds elapse or a brave admin uses xl.
Merge pull request #3 from TomMD/halvm

Only block if when waiting for a non-elapsed time.