Assigns a specified keymap matrix (ptr to FLASH) to a given keymap and modifier type.
*** adding virtual devices - called from keymap setup routine
**** asdf_virtual_init()
***** Clears the virtual device table for the keymap
***** initialize first entry in virtual device table to NULL
**** asdf_virtual_add_device()
***** Accept a virtual device code, a physical device code, a default function, and a default initial value, and create a new entry in the virtual device table.
***** increment the virtual device table pointer and set next (last) entry to NULL