Description of problem:

When guestfs_launch() returns -1, guestfs_last_error()
can return NULL.  This produces errors from the Perl
bindings such as:

  (null) at virt-rescue line 146.

where virt-rescue:146 is the line containing $g->launch()

Version-Release number of selected component (if applicable):

libguestfs 1.3.9

How reproducible:

--?

Steps to Reproduce:
1. Run virt-rescue
2. In another window, kill the qemu process
3.
  
Actual results:

(null) at virt-rescue line 146.

Expected results:

A more useful error message.

Additional info: