golang: Fix for out-of-tree builds

RWMJ:

This is half of a downstream Debian patch to fix out-of-tree builds
for golang.
fuse: Add guestmount --fd option (RHBZ#1100498).

This implements the guestmount --fd option to allow you to run
guestmount captive under another process (typically using
`guestmount --fd=<FD> --no-fork').

See: https://bugzilla.redhat.com/show_bug.cgi?id=1100498
fuse: Add a test of guestmount --fd option (RHBZ#1100498).