New definitions for MMC XPT layer

Added the following definitions:
      * XPT_MMC_IO: MMC I/O operation;
      * struct ccb_mmcio: describes MMC I/O operation.
Handle XPT_MMC_IO in xpt_action_default
Added an interrupt handler to test async events

The handler is called from callout(9) subsystem when certain MMC
requests are accepted by the driver.
Huge changes in MMC XPT

The most important: mmcprobe_done really works now :-)
State correctly gets transitioned, at the end (in PROBE_DONE)
mmcprobe device gets correctly destroyed and pass(4) is attached
to the SIM.