MIPS: Retire kvm paravirt

paravirt machine was introduced for Cavium's partial virtualization
technology, however, it's host side support and QEMU support never
landed in upstream.

As Cavium was acquired by Marvel and they have no intention to maintain
their MIPS product line, also paravirt is unlikely to be utilized by
community users, it's time to retire it if nobody steps in to maintain
it.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>