vmbus_server: Move modify_connection into restore and post_restore into start

[will-j-wright]

In order to more closely map OpenVMM's restore process onto Hyper-V, remove the need to manually call ServerWithNotifier::post_restore after Server::restore.

• Move Server::restore to ServerWithNotifier so we can call modify_connection,
• Move the modify_connection request from post_restore into restore,
• Rename post_restore to revoke_unclaimed_channels.
• Remove VmbusRequest::PostRestore and move the revoke_unclaimed_channels call into Start,
• Remove ModifyConnectionRequest::force, because ServerWithNotifier::restore_channel is now called after modify_connection, meaning there will be no open channels before the latter call.

[will-j-wright]

I considered adding a flag to ServerTaskInner to track whether or not this is the first start after a restore, but I think the overhead of running revoke_unclaimed_channels on each start isn't enough to really worry about. If anyone thinks otherwise, let me know

[jstarks]

I will remove post_restore from the openvmm state machine once you get this in.

[SvenGroot]

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