[cloud] Add utility to read INT13CON partition in Google Compute Engine

[cloud] Add utility to read INT13CON partition in Google Compute Engine

Following the example of aws-int13con, add a utility that can be used
to read the INT13 console log from a used iPXE boot disk in Google
Compute Engine.

There seems to be no easy way to directly read the contents of either
a disk image or a snapshot in Google Cloud.  Work around this
limitation by creating a snapshot and attaching this snapshot as a
data disk to a temporary Linux instance, which is then used to echo
the INT13 console log to the serial port.

Signed-off-by: Michael Brown <mcb30@ipxe.org>