KVM: add kvm_para_available to asm-generic/kvm_para.h

According to include/uapi/linux/kvm_para.h architectures should define
kvm_para_available, so add an implementation to asm-generic/kvm_para.h
which just returns false.

This fixes intel8x0.c build failure on mips with KVM enabled.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Gleb Natapov <gleb@redhat.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Gleb Natapov <gleb@redhat.com>