pci: vfio_user: Update all fields in MmioRegion on map

When mapping the region into the guest ensure that all the fields are
updated correctly as the unmap code path checks that they are set.

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

pci/src/vfio_user.rs

@@ -385,6 +385,7 @@ impl VfioUserPciDevice {
 
                 mmio_region.mem_slot = Some(slot);
                 mmio_region.host_addr = Some(host_addr as u64);
+                mmio_region.mmap_size = Some(mmio_region.length as usize);
             }
         }