conf: put data for network <forward> element into its own struct

conf: put data for network <forward> element into its own struct

The attributes of a <network> element's <forward> element were
previously stored directly in the virNetworkDef object, but
virNetworkUpdateForward() needs to operate on a <forward> in
isolation, so this patchs pulls out all those attributes into a
separate virNetworkForwardDef struct (and shortens their names
appropriately). This new object is contained in the virNetworkDef, not
pointed to by it, so there is no extra memory management.

This patch makes no functional changes, it only changes, e.g.,
"nForwardIfs" to "forward.nifs".