Setting the xen vm device id on vm record

Currently, while setting the device_id, the hypervisor
version is being incorrectly handled since the method
get_major_minor_version expects either an int, float,
or string format. However, the xen hypervisor
version is stored in the session product_version as a tuple.
This fix correctly uses the version to set the appropriate
device_id on the vm record. Also, removing the method
get_major_minor_version as it is not needed anymore.

Closes-bug: #1260178

Change-Id: I6b1353cdb7cc856b34f02e3d3679c142ba032533
Merge "Setting the xen vm device id on vm record"