mips/kvm: Use KVM_REG_MIPS and proper size indicators for *_ONE_REG

The API requires that the GET_ONE_REG and SET_ONE_REG ioctls have this
extra information encoded in the register identifiers.

Signed-off-by: David Daney <david.daney@cavium.com>
Signed-off-by: Gleb Natapov <gleb@redhat.com>