bridge: Fix generation of dnsmasq's --dhcp-hostsfile option

bridge: Fix generation of dnsmasq's --dhcp-hostsfile option

I added a host definition to a network definition:

<network>
  <name>Lokal</name>
  <uuid>2074f379-b82c-423f-9ada-305d8088daaa</uuid>
  <bridge name='virbr1' stp='on' delay='0' />
  <ip address='192.168.180.1' netmask='255.255.255.0'>
    <dhcp>
      <range start='192.168.180.128' end='192.168.180.254' />
      <host mac='23:74:00:03:42:02' name='somevm' ip='192.168.180.10' />
    </dhcp>
  </ip>
</network>

But due to the wrong if-statement the argument --dhcp-hostsfile doesn't get
added to the dnsmasq command. The patch below fixes it for me.