Record time of guest launch.

Record time of guest launch.

The guest handle field start_t was previously used (when we
had the wait_ready call), but had fallen into disuse.  Note
that it could never be accessed through the API.

Rename this field as launch_t, convert it to a timeval, and
use it to measure the time since guestfs_launch was called
so that we can start profiling guest launch.