net: qemu: Adding support for User Networking

Added support for Qemu User Networking and tested with
qemu_x86 model. The support is kept simple assuming that
the TAP interface will always be preferred for more
sophisticated / practical use cases.

QEMU User Networking is implemented using "slirp", which
provides a full TCP/IP stack within QEMU and uses that
stack to implement a virtual NAT'd network. As this
support is built into QEMU, it can be used with any model
and requires no admin privileges on the host machine,
unlike TAP.

Added documentation to facilitate the user.

Signed-off-by: Bilal Wasim <bilalwasim676@gmail.com>