logging: inhibit virtlogd shutdown while log files are open

logging: inhibit virtlogd shutdown while log files are open

The virtlogd daemon is launched with a 30 second timeout for
unprivileged users. Unfortunately the timeout is only inhibited
while RPC clients are connected, and they only connect for a
short while to open the log file descriptor. We need to hold
an inhibition for as long as the log file descriptor itself
is open.

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