Drivers: hv: Cleanup the guest ID computation

The current guest ID string in use in vmbus driver does not conform
to the MSFT guidelines on guest ID. MSFT currently does not specify
Linux specific guidelines. MSFT however has plans to publish Linux
specific guidelines. This implementation conforms to the yet unpublished
Linux specific guidelines for guest ID. This implementation also broadly
conforms to the current guidelines as well.

Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Reviewed-by: Haiyang Zhang <haiyangz@microsoft.com>
Cc: Bjørn Mork <bjorn@mork.no>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>