mips/kvm: Support signed 64-bit KVM registers

mips/kvm: Support signed 64-bit KVM registers

Rename kvm_mips_{get,put}_one_reg64() to kvm_mips_{get,put}_one_ureg64()
since they take an int64_t pointer, and add separate signed 64-bit
accessors. These will be used for double precision floating point
registers.

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>