Turns out that it's just a file kept in .gvm/cards.
$ mkdir ~/.gvm/cards $ echo -e "/media/mmc1\nRW" > ~/.gvm/cards/0
EDIT: If you want, check output of HostFSTest.prc inside HostFSTest.zip with card enabled. See the code in testHostFS() in src/AppMain.c. Basically all I got is Hello World from HostTraceOutput, nothing more.