util: avoid glib event loop workaround where possible

util: avoid glib event loop workaround where possible

I previously did a workaround for a glib event loop race
that causes crashes:

  commit 0db4743645b7a0611a3c0687f834205c9956f7fc
  Author: Daniel P. Berrangé <berrange@redhat.com>
  Date:   Tue Jul 28 16:52:47 2020 +0100

    util: avoid crash due to race in glib event loop code

it turns out that the workaround has a significant performance
penalty on I/O intensive workloads. We thus need to avoid the
workaround if we know we have a new enough glib to avoid the
race condition.

Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Tested-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>