migration: introduce a new QEMUFile impl based on QIOChannel

migration: introduce a new QEMUFile impl based on QIOChannel

Introduce a new QEMUFile implementation that is based on
the QIOChannel objects. This impl is different from existing
impls in that there is no file descriptor that can be made
available, as some channels may be based on higher level
protocols such as TLS.

Although the QIOChannel based implementation can trivially
provide a bi-directional stream, initially we have separate
functions for opening input & output directions to fit with
the expectation of the current QEMUFile interface.

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-Id: <1461751518-12128-9-git-send-email-berrange@redhat.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>