proxy volume monitor: Don't disable exit_on_close on session bus
proxy volume monitor: Don't disable exit_on_close on session bus

We're on shared session bus, let's exit when connection closes
(usually tied to running desktop session).

https://bugzilla.gnome.org/show_bug.cgi?id=687074

gicmo/gvfs,xkahn/gvfs-cmis,xkahn/gvfs-cmis,halfline/gvfs,gicmo/gvfs,halfline/gvfs,halfline/gvfs,gicmo/gvfs,xkahn/gvfs-cmis,gicmo/gvfs,xkahn/gvfs-cmis