mips/kvm: Sign extend registers written to KVM

In case we're running on a 64-bit host, be sure to sign extend the
general purpose registers and hi/lo/pc before writing them to KVM, so as
to take advantage of MIPS32/MIPS64 compatibility.

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>
Cc: kvm@vger.kernel.org
Cc: qemu-stable@nongnu.org
Message-Id: <1429871214-23514-3-git-send-email-james.hogan@imgtec.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>