FUSE filesystem support.

FUSE filesystem support.

This implements FUSE filesystem support so that any libguestfs-
accessible disk image can be mounted as a local filesystem.

Note: file writes (ie. write(2) system call) is not yet implemented.

The API needs more test coverage, particularly lesser-used system
calls.

The big unresolved issue is UID/GID mapping between guest filesystem
IDs and the host.  It's not easy to automate this because you need
extra details about the guest itself in order to get to its
UID->username map (eg. /etc/passwd from the guest).