Revert "session: make spice_session_get_webdav_server() public"

This reverts commit 4b9092b9.

Instead of publishing the function to access the PhodavServer
from spice-gtk-session.c, install a new read-only property
"webdav-server" and use g_object_get() to retrieve it.

Signed-off-by: Jakub Janků <jjanku@redhat.com>