virNetworkDHCPLeaseTimeDefParseXML: Output error when 'expiry' can't be parsed

virStrToLong_ul doesn't report it's own error.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1918674


Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Laine Stump <laine@redhat.com>