mips/kvm: Use ENOIOCTLCMD to indicate unimplemented ioctls.

The Linux Way is to return -ENOIOCTLCMD to the vfs when an
unimplemented ioctl is requested.  Do this in kvm_mips instead of a
random mixture of -ENOTSUPP and -EINVAL.

Signed-off-by: David Daney <david.daney@cavium.com>
Acked-by: Sanjay Lal <sanjayl@kymasys.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>