Remove the assumption that every QEMUFile implementation has
a file descriptor available by introducing a new function
in QEMUFileOps to change the blocking state of a QEMUFile.

If not set, it will fallback to the original code using
the get_fd method.
Reviewed-by: NDr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>
Reviewed-by: NJuan Quintela <quintela@redhat.com>
Message-Id: <1461751518-12128-7-git-send-email-berrange@redhat.com>
Signed-off-by: NAmit Shah <amit.shah@redhat.com>