DM: adapt to the new VHM request state transitions

This is the counterpart in DM to the VHM request state update in the
hypervisor. Major changes include:

    * Remove accesses to the obsolete 'valid' member.
    * Access the 'processed' member using atomic operations.
    * Sync the documentation on vhm_request.

In addition, the new state transition also requires a VHM request to be always
handled properly, as there is no 'FAILED' state any more. Instead of crashing
the device model (and thus the UOS as well), the device model should return all
1s or ignore the request when it is to load from or store to an invalid address,
respectively.

Note: there is an issue in vm_system_reset() and vm_suspend_resume() where
completed VHM requests are not properly notified, causing the hypervisor to
complain as it sees uncompleted requests while trying to create a new one. This
issue will be resolved in a separate patch.

v1 -> v2:

    * Use macro-defined constants for the default values for invalid PIO/MMIO
      reads.
    * Change the return type of vmexit_handler_t in DM to void as the return
      values are no longer necessary.
    * Remove VM_EXITCODE that are no longer used.

Tracked-On: #875
Signed-off-by: Junjie Mao <junjie.mao@intel.com>
Acked-by: Yu Wang <yu1.wang@intel.com>

devicemodel/include/dm.h

@@ -29,8 +29,6 @@
 #ifndef	_DM_H_
 #define	_DM_H_
 
-#define	VMEXIT_CONTINUE		(0)
-#define	VMEXIT_ABORT		(-1)
 #include <stdbool.h>
 #include "types.h"
 #include "vmm.h"