mips/kvm: Fix ABI for use of 64-bit registers.

All registers are 64-bits wide, 32-bit guests use the least
significant portion of the register storage fields.

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