conf: reorganize virNetworkDHCPDefParseXML

conf: reorganize virNetworkDHCPDefParseXML

This makes the range and static host array management in
virNetworkDHCPDefParseXML() more similar to what is done in
virNetworkDefUpdateIPDHCPRange() and virNetworkDefUpdateIPDHCPHost() -
they use VIR_APPEND_ELEMENT rather than a combination of
VIR_REALLOC_N() and separate incrementing of the array size.

The one functional change here is that a memory leak of the contents
of the last (unsuccessful) virNetworkDHCPHostDef was previously leaked
in certain failure conditions, but it is now properly cleaned up.