Fix qemu boot pause

qemu with "-daemonize" option is not working well with POSIX::Spawn, specifically with posix_spawn() C function.

Symptom is that a zombie process comes up when "qemu -daemonize" is run. Internal waitpid() of POSIX::Spawn waits the process forever so hva seems to hang at the position. But the standard spawn() method does not cause this issue.

As for tentative fix, POSIX::Spawn is patched to disable C extention and work with Process#spawn instead.