MIPS: KVM: Emulate generic QEMU machine on r6 T&E

Default the guest PRId register to represent a generic QEMU machine
instead of a 24kc on MIPSr6. 24kc isn't supported by r6 Linux kernels.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim KrÄmář <rkrcmar@redhat.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Cc: kvm@vger.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>