vfio: Extend the device migration protocol with RUNNING_P2P

The RUNNING_P2P state is designed to support multiple devices in the same
VM that are doing P2P transactions between themselves. When in RUNNING_P2P
the device must be able to accept incoming P2P transactions but should not
generate outgoing P2P transactions.

As an optional extension to the mandatory states it is defined as
in between STOP and RUNNING:
   STOP -> RUNNING_P2P -> RUNNING -> RUNNING_P2P -> STOP

For drivers that are unable to support RUNNING_P2P the core code
silently merges RUNNING_P2P and RUNNING together. Unless driver support
is present, the new state cannot be used in SET_STATE.
Drivers that support this will be required to implement 4 FSM arcs
beyond the basic FSM. 2 of the basic FSM arcs become combination
transitions.

Compared to the v1 clarification, NDMA is redefined into FSM states and is
described in terms of the desired P2P quiescent behavior, noting that
halting all DMA is an acceptable implementation.

Link: https://lore.kernel.org/all/[email protected]
Signed-off-by: Jason Gunthorpe <[email protected]>
Tested-by: Shameer Kolothum <[email protected]>
Reviewed-by: Kevin Tian <[email protected]>
Reviewed-by: Alex Williamson <[email protected]>
Signed-off-by: Yishai Hadas <[email protected]>
Signed-off-by: Leon Romanovsky <[email protected]>

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