doc: improve VNC/SPICE password documentation
doc: improve VNC/SPICE password documentation

If the passwd attribute is set to empty string it disables VNC/SPICE
access to the guest.

Signed-off-by: Pavel Hrdina <d4772d05997b8abf035041e3b4f4996380ea7e7a@redhat.com>

datto/libvirt,olafhering/libvirt,jfehlig/libvirt,eskultety/libvirt,fabianfreyer/libvirt,zippy2/libvirt,taget/libvirt,jardasgit/libvirt,VenkatDatta/libvirt,crobinso/libvirt,eskultety/libvirt,jardasgit/libvirt,andreabolognani/libvirt,fabianfreyer/libvirt,zippy2/libvirt,eskultety/libvirt,libvirt/libvirt,datto/libvirt,taget/libvirt,eskultety/libvirt,fabianfreyer/libvirt,olafhering/libvirt,zippy2/libvirt,jardasgit/libvirt,taget/libvirt,datto/libvirt,taget/libvirt,fabianfreyer/libvirt,nertpinx/libvirt,jfehlig/libvirt,jfehlig/libvirt,datto/libvirt,taget/libvirt,datto/libvirt,nertpinx/libvirt,VenkatDatta/libvirt,libvirt/libvirt,nertpinx/libvirt,andreabolognani/libvirt,fabianfreyer/libvirt,zippy2/libvirt,olafhering/libvirt,nertpinx/libvirt,andreabolognani/libvirt,jardasgit/libvirt,VenkatDatta/libvirt,nertpinx/libvirt,libvirt/libvirt,crobinso/libvirt,andreabolognani/libvirt,jfehlig/libvirt,eskultety/libvirt,olafhering/libvirt,VenkatDatta/libvirt,libvirt/libvirt,andreabolognani/libvirt,jardasgit/libvirt,crobinso/libvirt,crobinso/libvirt,VenkatDatta/libvirt