kvm tools: Support multiple net devices

This patch adds support for multiple network devices. The command line syntax
changes to the following:

	--network/-n [mode=tap/user/none][,guest_ip=ip][,host_ip=
ip][,guest_mac=mac][,script=file]

Each of the parameters is optional, and the config defaults to a TAP based
networking with a sequential MAC.

Acked-by: Asias He <asias.hejun@gmail.com>
Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>