uml: Exit status 1 is a normal exit code for vmlinux subprocess.

Note you need to apply these patches to the Linux kernel
to get User-Mode Linux to work with libguestfs:

http://marc.info/?l=user-mode-linux-devel&m=137682544009451&w=2
http://marc.info/?l=user-mode-linux-devel&m=137682543809449&w=2
http://marc.info/?l=user-mode-linux-devel&m=137682544209452&w=2
http://marc.info/?l=user-mode-linux-devel&m=137682543809448&w=2
uml: Remove kill_vmlinux subprocess, problem fixed upstream.

Note you need to apply these patches to the Linux kernel to get
User-Mode Linux to work with libguestfs:

http://marc.info/?l=user-mode-linux-devel&m=137682544009451&w=2
http://marc.info/?l=user-mode-linux-devel&m=137682543809449&w=2
http://marc.info/?l=user-mode-linux-devel&m=137682544209452&w=2
http://marc.info/?l=user-mode-linux-devel&m=137682543809448&w=2