followup to disk_utils.count_mbr_partitions()
followup to disk_utils.count_mbr_partitions()

This addresses a couple of issues with disk_utils.count_mbr_partitions()
and is a followup to I7042cd12775b0d715c15f466db2d4c5dc9e1a60a.

This fixes a typo as well as adds the device to the error string.

Change-Id: I010b8a8ebb34bf776ea22dc5fed13fe868b0d794
Related-Bug: #1629926

openstack/ironic-lib,NaohiroTamura/ironic-lib