chardev: add Chardev.gcontext field

It caches the gcontext that is used to poll the chardev IO.  Before this
patch, we only passed it in via chr_update_read_handlers().  However
that may not be enough if the char backend is disconnected and
reconnected afterward.  There are chardev codes that still assumed the
context be NULL (which is the main context).  Will fix that up in
following up patches.

Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <1505975754-21555-3-git-send-email-peterx@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>