chardev: Shorten references into ChardevBackend

An upcoming patch will alter how simple unions, like ChardevBackend,
are laid out, which will impact all lines of the form 'backend->u.XXX'
(expanding it to the longer 'backend->u.XXX.data').  For better
legibility in that patch, and less need for line wrapping, it's better
to use a temporary variable to reduce the effect of a layout change to
just the variable initializations, rather than every reference within
a ChardevBackend.  It doesn't hurt that this also makes the code more
consistent: some clients touched here already had a temporary variable
but weren't using it.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-By: Daniel P. Berrange <berrange@redhat.com>
Message-Id: <1457021813-10704-6-git-send-email-eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>