CONFIG_UML_NET is not available for thedefaultarchitecture x86.
Result is shown for architecture um

Virtual network device

modulename: net.ko

configname: CONFIG_UML_NET

Linux Kernel Configuration
└─>UML Network Devices
└─>Virtual network device
In linux kernel since version 2.6.12  
While the User-Mode port cannot directly talk to any physical
hardware devices, this choice and the following transport options
provide one or more virtual network devices through which the UML
kernels can talk to each other, the host, and with the host's help,
machines on the outside world.

For more information, including explanations of the networking and
sample configurations, see
<http://user-mode-linux.sourceforge.net/old/networking.html>.

If you'd like to be able to enable networking in the User-Mode
linux environment, say Y; otherwise say N. Note that you must
enable at least one of the following transport options to actually
make use of UML networking.

source code:
depends
CONFIG_NET

is selected by
CONFIG_UML_NET_PCAP