Fix SteamOSBackend isValid check.

Since QDBusInterface isValid is not a good test of whether a dbus
service exists or not (according to QDBusAbstractInterface isValid
documentation) check if the service file is installed on the disk
instead.
Fixes spurious SteamOSBackend telling Discover that it is invalid
and missing updates, etc.

(cherry picked from commit b47d041a)