qemu: Enable disconnecting SPICE clients without changing password

qemu: Enable disconnecting SPICE clients without changing password

Libvirt updates the configuration of SPICE server only when something
changes. This is unfortunate when the user wants to disconnect a
existing spice session when the connected attribute is already
"disconnect".

This patch modifies the conditions for calling the password updater to
be called when nothing changes, but the connected attribute is already
"disconnect".
(cherry picked from commit e0f0131d33e283f1b76c05eb7bef4bbbb3f2fd72)