conf: domain: Add helper infrastructure for new connection close callbacks

conf: domain: Add helper infrastructure for new connection close callbacks

The new connect close callbacks for domains will be represented by a
virObject associated with the domain object itself.

To simplify handling the pointer to the close callback data will be done
by an immutable pointer allocated directly when allocating the
corresponding virDomainObj struct.

This patch adds the 'closecallbacks' field to virDomainObj and a
corresponding callback to allocate it into virDomainXMLOption.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>