inventory: Fix is_coming_from_pxe() method

Instead of check if a request is coming from PXE by IP address, check by
OSNAME value send in XML inventory.

is_coming_from_pxe() checked if IP send was the same than IP of
package-server defined in package-server.ini.

It's broken by design because this IP can be different (PXE send
127.0.0.1 and IP defined in package-server.ini was 0.0.0.0)

A OSNAME XML inventory value sent by PXE is always:
 * 'Unknown operating system (PXE network boot inventory)'