Move filedescriptor parsing code from net.c into qemu_parse_fd()

Move the parsing of a filedescriptor into a common function
qemu_parse_fd() so others can use it as well. Have net.c call this
function.

v2:
 - moving qemu_parse_fd into cutils.c

Signed-off-by: Stefan Berger <stefanb@linux.vnet.ibm.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

cutils.c

@@ -415,3 +415,15 @@ int64_t strtosz(const char *nptr, char **end)
 {
     return strtosz_suffix(nptr, end, STRTOSZ_DEFSUFFIX_MB);
 }
+
+int qemu_parse_fd(const char *param)
+{
+    int fd;
+    char *endptr = NULL;
+
+    fd = strtol(param, &endptr, 10);
+    if (*endptr || (fd == 0 && param == endptr)) {
+        return -1;
+    }
+    return fd;
+}