networkRunHook: Run hook only if possible

networkRunHook: Run hook only if possible

Currently, networkRunHook() is called in networkAllocateActualDevice and
friends. These functions, however, doesn't necessarily work on networks,
For example, if domain's interface is defined in this fashion:

    <interface type='bridge'>
      <mac address='52:54:00:0b:3b:16'/>
      <source bridge='virbr1'/>
      <model type='rtl8139'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x09' function='0x0'/>
    </interface>

The networkAllocateActualDevice jumps directly onto 'validate' label as
the interface is not type of 'network'. Hence, @network is left
initialized to NULL and networkRunHook(network, ...) is called. One of
the things that the hook function does is dereference @network. Soupir.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>