parallels: fix libvirt crash if parallelsNetworkOpen fails

parallels: fix libvirt crash if parallelsNetworkOpen fails

If, by any reason, parallelsNetworkOpen fails it dereferences
newly allocated privconn->networks via virObjectUnref, which in
turn deallocates its memory.
Subsequent call of parallelsNetworkClose calls virObjectUnref
that leads to double memory free. To prevent this we should zero
privconn->networks to make all subsequent virObjectUnref be safe.

Signed-off-by: Maxim Nestratov <mnestratov@parallels.com>