dm: add string convert API

As function sscanf is banned, to get value from parameter buffer,strto*
is recommended. To reduce the inspection code when using strto*, it's
better to use a string convert API.

Usage:
    For virtio-blk, it has parameters:
        range=<start lba>/<subfile size>
    sscanf:
        if (sscanf(cp, "range=%ld/%ld", &sub_file_start_lba,
                &sub_file_size) == 2)
            sub_file_assign = 1;
    string API:
        if (strsep(&cp, "=") &&
                !dm_strtol(cp, &cp, 10, &sub_file_start_lba) &&
                *cp == '/' &&
                !dm_strtol(cp + 1, &cp, 10, &sub_file_size))
            sub_file_assign = 1;

Tracked-on: #1496
Signed-off-by: Conghui Chen <conghui.chen@intel.com>
Reviewed-by: Shuo Liu <shuo.a.liu@intel.com>
Acked-by: Yin Fengwei <fengwei.yin@intel.com>

devicemodel/Makefile

@@ -56,6 +56,9 @@ LIBS += -luuid
 LIBS += -lusb-1.0
 LIBS += -lacrn-mngr
 
+# lib
+SRCS += lib/dm_string.c
+
 # hw
 SRCS += hw/block_if.c
 SRCS += hw/usb_core.c