conf: Remove the actual hostdev when removing a network

conf: Remove the actual hostdev when removing a network

Commit 50348e6edfa reused the code to remove the hostdev portion of a
network definition on multiple places but forgot to take into account
that sometimes the "actual" network is passed and in some cases the
parent of that.

This patch uses the virDomainNetGetActualHostdev() helper to acquire the
correct pointer all the time while removing the hostdev portion from the
list.