vfio/virtio: Add support for the basic live migration functionality

Add support for basic live migration functionality in VFIO over
virtio-net devices, aligned with the virtio device specification 1.4.

This includes the following VFIO features:
VFIO_MIGRATION_STOP_COPY, VFIO_MIGRATION_P2P.

The implementation registers with the VFIO subsystem using vfio_pci_core
and then incorporates the virtio-specific logic for the migration
process.

The migration follows the definitions in uapi/vfio.h and leverages the
virtio VF-to-PF admin queue command channel for execution device parts
related commands.

Additional Notes:
-----------------
The kernel protocol between the source and target devices contains a
header with metadata, including record size, tag, and flags.

The record size allows the target to recognize and read a complete image
from the source before passing the device part data. This adheres to the
virtio device specification, which mandates that partial device parts
cannot be supplied.

The tag and flags serve as placeholders for future extensions of the
kernel protocol between the source and target, ensuring backward and
forward compatibility.

Both the source and target comply with the virtio device specification
by using a device part object with a unique ID as part of the migration
process. Since this resource is limited to a maximum of 255, its
lifecycle is confined to periods with an active live migration flow.

According to the virtio specification, a device has only two modes:
RUNNING and STOPPED. As a result, certain VFIO transitions (i.e.,
RUNNING_P2P->STOP, STOP->RUNNING_P2P) are treated as no-ops. When
transitioning to RUNNING_P2P, the device state is set to STOP, and it
will remain STOPPED until the transition out of RUNNING_P2P->RUNNING, at
which point it returns to RUNNING. During transition to STOP, the virtio
device only stops initiating outgoing requests(e.g. DMA, MSIx, etc.) but
still must accept incoming operations.

Signed-off-by: Yishai Hadas <yishaih@nvidia.com>
Link: https://lore.kernel.org/r/20241113115200.209269-6-yishaih@nvidia.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

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