Add assignable devices xml

This XML's schema is defined as assignable_devices.xsd in
packages/modules/Virtualization, and contains a list of devices which
can be assigned to a VM. Virtualization Service will read this file and
return the list to users.

Bug: 297313212
Test: vm info
Change-Id: I894df4804cdfd1251871c7f2b1ac903c9f1ea55d