Implement progress messages in the daemon and library.

Implement progress messages in the daemon and library.

This implements progress notification messages in the daemon, and
adds a callback in the library to handle them.

No calls are changed so far, so in fact no progress messages can
be generated by this commit.

For more details, see:
https://www.redhat.com/archives/libguestfs/2010-July/msg00003.html
https://www.redhat.com/archives/libguestfs/2010-July/msg00024.html