nwfilter: remove pointless virNWFilterHashTable struct

nwfilter: remove pointless virNWFilterHashTable struct

The virNWFilterHashTable struct only contains a single virHashTable
member since

  commit 293d4fe2f11db98c91175525056c8883725d4b22
  Author: Daniel P. Berrange <berrange@redhat.com>
  Date:   Mon Mar 24 16:35:23 2014 +0000

    Remove pointless storage of var names in virNWFilterHashTable

Thus, this struct wrapper adds no real value over just using the
virHashTable directly, but brings the complexity of needing to derefence
the hashtable to call virHash* APIs, and adds extra memory allocation
step.

To minimize code churn this just turns virNWFilterHashTable into a
typedef aliases virHashTable.

Reviewed-by: Jiri Denemark <jdenemar@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>