manual: update webdav virt-manager section
manual: update webdav virt-manager section

virt-manager can add webdav channel for a while now.

Signed-off-by: Marc-André Lureau <a6f2b960a4aca61381fc650a67901b2ac8e83e6d@gmail.com>
Acked-by: Christophe Fergeau <05b95054a663c388a4e33d95828cb30f0370f8ef@redhat.com>

fgouget/spice,ya-mouse/spice,fgouget/spice,fgouget/spice,ya-mouse/spice,SPICE/spice,fgouget/spice,ya-mouse/spice,SPICE/spice,SPICE/spice