domain_conf: cleanup virDomainGraphicsGetListen

domain_conf: cleanup virDomainGraphicsGetListen

Removes the check for graphics type, it's not a public API and developer
know what he's doing and this check makes no sense.  It also removes
the ability to allocate a new array if there is none.  This was used by
the virDomainGraphicsListenAdd* functions and isn't used anymore.

This is now a simple getter with simple check for listens array presence
and whether the index in out of bounds.

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