util: set missing data length in virSocketAddrPrefixToNetmask()

util: set missing data length in virSocketAddrPrefixToNetmask()

This fixes a bug that has been present since the original version of
the function was pushed in commit 1ab80f3 on Nov. 26 2010 (by me). The
virSocketAddr::len was not being set.

Apparently until now we were always calling
virSocketAddrPrefixToNetmask with virSocketAddr object that was
already (coincidentally) initialized for the proper address family,
but the bug became apparent when trying to use it to fill in an
otherwise uninitialized object.

Signed-off-by: Laine Stump <laine@laine.org>
Reviewed-by: John Ferlan <jferlan@redhat.com>