Drivers: hv: vmbus: Move the definition of generate_guest_id()

As part of the effort to separate out architecture specific code, move the
definition of generate_guest_id() to x86 specific header file.

Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>