qemu: support netdevs from <forward mode='hostdev'> networks

qemu: support netdevs from <forward mode='hostdev'> networks

For network devices allocated from a network with <forward
mode='hostdev'>, there is a need to add the newly minted hostdev to
the hostdevs array.

In this case we also need to call qemuPrepareHostDevices just for this
one device, as the standard call to initialize all the hostdevs that
were defined directly in the domain's configuration has already been
made by the time we allocate a device from a libvirt network, and thus
have something that needs initializing.

Signed-off-by: Shradha Shah <sshah@solarflare.com>