Since MIPS/KVM only supports hardware virtualization using MIPS VZ,
do not enable KVM blindly.  Use a new Kconfig symbol CPU_SUPPORTS_VZ
and do not enable it for R2 processors.

Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>