bhyve: fix build with gcc48
    
    Build with gcc 4.8 fails with:
    
    bhyve/bhyve_monitor.c: In function 'bhyveMonitorIO':
    bhyve/bhyve_monitor.c:51:18: error: missing initializer for field 'tv_sec' of 'const struct timespec' [-Werror=missing-field-initializers]
         const struct timespec zerowait = {};
    
    Explicitly initialize zerowait to fix the build.