Add a prefix to output when tracing (RHBZ#673479).

Add a prefix to output when tracing (RHBZ#673479).

Also separate the call and return lines so that everything can be
easily 'grepped' from debug output.  The trace output now looks like
this:

$ guestfish -x -N fs exit
libguestfs: trace: is_config
libguestfs: trace: is_config = 1
libguestfs: trace: add_drive "test1.img"
libguestfs: trace: add_drive = 0
libguestfs: trace: is_config
libguestfs: trace: is_config = 1
libguestfs: trace: launch
libguestfs: trace: launch = 0
libguestfs: trace: part_disk "/dev/sda" "mbr"
libguestfs: trace: part_disk = 0
&c.