virlease: Allow infinite lease expiry time

virlease: Allow infinite lease expiry time

When adding a new lease by our leaseshelper then virLeaseNew() is
called. Here, we check for DNSMASQ_LEASE_EXPIRES environment
variable which is the expiration time for the lease. For infinite
lease time the value is zero. However, our code is not prepared
for that and adds "expiry-time" into the JSON file only if lease
expiry time is non-zero. This breaks the assumption that the
"expiry-time" attribute is always present (as can be seen in
virLeaseReadCustomLeaseFile() and virLeasePrintLeases()).

Store "expiry-time" always.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>