cl/396457068: Teach fifo_agent to connect to an existing enclave.
cl/396457068: Teach fifo_agent to connect to an existing enclave.

TESTED:
$ mount -t ghost ghost /sys/fs/ghost
$ echo "create 1" > /sys/fs/ghost/ctl
$ echo "0-5" > /sys/fs/ghost/enclave_1/cpulist
$ fifo_agent --enclave=/sys/fs/ghost/enclave_1

google/ghost-userspace,google/ghost-userspace,google/ghost-userspace,google/ghost-userspace