conf: eliminate redundant VIR_ALLOC of 1st element of network DNS hosts.

conf: eliminate redundant VIR_ALLOC of 1st element of network DNS hosts.

virNetworkDNSHostsDefParseXML was calling VIR_ALLOC(def->hosts) if
def->hosts was NULL. This is a waste of time, though, since
VIR_REALLOC_N is called a few lines further down, prior to any use of
def->hosts. (initializing def->nhosts to 0 is also redundant, because
the newly allocated memory will always be cleared to all 0's anyway).