vhost-net: revamp configure logic

vhost-net: revamp configure logic

Detect all invalid configurations (e.g. mingw32 with vhost-user,
non-Linux with vhost-kernel).  As a collateral benefit, all vhost-kernel
backends can be now disabled if one wants to reduce the attack surface.

Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>
Reviewed-by: Thomas Huth <[email protected]>
Message-Id: <1543851204[email protected]>
Message-Id: <1550165756[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>