udev-rules: accept cvm CONST matches

udev-rules: accept cvm CONST matches

CONST{cvm} is documented and already handled when udev rules are
executed, but the parser rejected it before the rule could be used.

Include cvm in the supported CONST key list.

Follow-up for 6e2e83b48734e86992cbdbb329c48cc066cf7c96.

Reproducer:

$ printf 'CONST{cvm}=="none", NAME="x"\n' > /tmp/codex-cvm.rules
$ udevadm verify /tmp/codex-cvm.rules
/tmp/codex-cvm.rules:1 Invalid attribute for CONST.
/tmp/codex-cvm.rules: udev rules check failed.

1 udev rules files have been checked.
  Success: 0
  Fail:    1