network: new function networkGetActualType

network: new function networkGetActualType

There are times when it's necessary to learn the actual type of a
network connection before any resources have been allocated
(e.g. during qemuProcessPrepareDomain()), but in the past it was
necessary to call networkAllocateActualDevice() in order to have the
actual type filled in.

This new function returns the type of network that *will be* setup
once it actually happens, but without making any changes on the host.