udev: introduce CONST key name

udev: introduce CONST key name

Currently, there is no way to match against system-wide constants, such
as architecture or virtualization type, without forking helper binaries.
That potentially results in a huge number of spawned processes which
output always the same answer.

This patch introduces a special CONST keyword which takes a hard-coded
string as its key and returns a value assigned to that key. Currently
implemented are CONST{arch} and CONST{virt}, which can be used to match
against the system's architecture and virtualization type.