add support for dm verity target when mounting VPMem devices

Related work items: microsoft#388, microsoft#389, microsoft#393, microsoft#394, microsoft#395, microsoft#396, microsoft#397, microsoft#398, microsoft#399, microsoft#400, microsoft#401, microsoft#402, microsoft#403, microsoft#404, microsoft#405, microsoft#931, microsoft#973, microsoft#1001, microsoft#1003, microsoft#1004, microsoft#1005, microsoft#1006, microsoft#1007, microsoft#1009, microsoft#1010, microsoft#1012, microsoft#1013, microsoft#1014, microsoft#1015, microsoft#1016, microsoft#1017, microsoft#1019, microsoft#1021, microsoft#1022, microsoft#1024, microsoft#1025, microsoft#1027, microsoft#1028, microsoft#1029, microsoft#1030, microsoft#1031, microsoft#1033