mips/kvm: Implement Config CP0 registers

mips/kvm: Implement Config CP0 registers

Implement saving and restoring to KVM state of the Config CP0 registers
(namely Config, Config1, Config2, Config3, Config4, and Config5). These
control the features available to a guest, and a few of the fields will
soon be writeable by a guest so QEMU needs to know about them so as not
to clobber them on migration/savevm.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Leon Alrae <leon.alrae@imgtec.com>
Cc: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Leon Alrae <leon.alrae@imgtec.com>