vmbus_server: improve revoke and restore flows for proxy driver channels.

[SvenGroot]

This change makes some improvements to how revoke (particularly while terminating) and restore are handled for proxy driver channels. These are mainly aimed at situations where the proxy driver may maintain state for some open channels in between save/restore, though they should improve the general reliability as well.

• When the proxy task is being cancelled, only "next action" ioctls are aborted/prevented from issuing. All other ioctls are still allowed, so that state changes are fully propagated to the driver until the server terminates. It's now also possible to wait for proxy task completion.
• When the proxy driver revokes a channel, that is now handled inline using ChannelServerRequest::Revoke, so we can wait for its completion and immediately release the channel to the driver.
• Restoring a channel and its associated gpadls is now done in a single ioctl, to make it easier for the driver to distinguish restored gpadls from new ones, in case it had preserved the gpadl state for a channel.

When making these changes, I fixed a few other omissions in the restore path:

• All states that the driver should consider as a channel being open on restore are now properly handled.
• The proxy task will no longer mark a channel as restored if the device specified the "force new channel" flag.
• When saved in the disconnected state, properly restore any saved reserved channels.
• I also refactored how SavedState is used (within the confines of not being able to change its layout for compatibility), so Rust's type system can hopefully guide us into considering both connected and disconnected states in the future.

[github-actions]

At least one Petri test failed.

[will-j-wright]

Nice, I like the SavedStateData idea. A lot nicer than just randomly pub-ing fields :)

[benhillis]

LGTM.