virtio: replace restricted mem access flag with callback

Instead of having a global flag to require restricted memory access
for all virtio devices, introduce a callback which can select that
requirement on a per-device basis.

For convenience add a common function returning always true, which can
be used for use cases like SEV.

Per default use a callback always returning false.

As the callback needs to be set in early init code already, add a
virtio anchor which is builtin in case virtio is enabled.

Signed-off-by: Juergen Gross <[email protected]>
Tested-by: Oleksandr Tyshchenko <[email protected]> # Arm64 guest using Xen
Reviewed-by: Stefano Stabellini <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Juergen Gross <[email protected]>

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