Turn virLogSource into a struct instead of an enum

Turn virLogSource into a struct instead of an enum

As part of the goal to get away from doing string matching on
filenames when deciding whether to emit a log message, turn
the virLogSource enum into a struct which contains a log
"name". There will eventually be one virLogSource instance
statically declared per source file. To minimise churn in this
commit though, a single global instance is used.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>