add patch to avoid huge snapshot performance regression

add patch to avoid huge snapshot performance regression

Taking a snapshot became prohibitively slow because of the
migration_transferred_bytes() call in migration_rate_exceeded() [0].

This also applied to the async snapshot taking in Proxmox VE, so
work around the issue until it is fixed upstream.

[0]: https://gitlab.com/qemu-project/qemu/-/issues/1821

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>