hw/cxl: Fix background completion percentage calculation

hw/cxl: Fix background completion percentage calculation

The current completion percentage calculation does not account for the
relative time since the start of the background activity, this leads to
showing incorrect start percentage vs what has actually been completed.

This patch calculates the percentage based on the actual elapsed time since
the start of the operation.

Fixes: 221d2cfbdb53 ("hw/cxl/mbox: Add support for background operations")
Signed-off-by: Ajay Joshi <ajay.opensrc@micron.com>
Reviewed-by: Davidlohr Bueso <dave@stgolabs.net>
Link: https://lore.kernel.org/r/20240729102338.22337-1-ajay.opensrc@micron.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Message-Id: <20241014121902.2146424-3-Jonathan.Cameron@huawei.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>