hw/cxl: factor out calculation of sanitize duration from cmd_santize_overwrite

hw/cxl: factor out calculation of sanitize duration from cmd_santize_overwrite

Move the code of calculation of sanitize duration into function
for usability by other sanitize routines

Estimate times based on:
             https://pmem.io/documents/NVDIMM_DSM_Interface-V1.8.pdf

Signed-off-by: Davidlohr Bueso <dave@stgolabs.net>
Signed-off-by: Vinayak Holikatti <vinayak.kh@samsung.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Message-Id: <20250305092501.191929-5-Jonathan.Cameron@huawei.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>