domain_conf: introduce virDomainGraphicsListenAppendAddress

domain_conf: introduce virDomainGraphicsListenAppendAddress

This effectively removes virDomainGraphicsListenSetAddress which was
used only to change the address of listen structure and possible change
the listen type.  The new function will auto-expand the listens array
and append a new listen.

The old function was used on pre-allocated array of listens and in most
cases it only "add" a new listen.  The two remaining uses can access the
listen structure directly.

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>