virGetConnectGeneric: Only delegate existing identities

virGetConnectGeneric: Only delegate existing identities

Inside virt-qemu-run, just like in virsh for example, there is no
identity set in the current thread, so we should not try to set it,
otherwise things like connecting to other drivers might fail and on
top of that there is no error set so the user can't even see what's
wrong.

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=2000075
Signed-off-by: Martin Kletzander <mkletzan@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>