Replace UTF8_STRING with OEM_STRING

Replace UTF8_STRING with OEM_STRING

The UTF8 conversion functions treat UTF8_STRING as:
  struct {
      USHORT  Length;
      CHAR    Buffer[ANYSIZE_ARRAY];
  } UTF8_STRING;

But UTF8_STRING is defined by later WDKs as:
  struct {
      USHORT  Length;
      USHORT  MaximumLength;
      PCHAR   Buffer;
  } UTF8_STRING;

This difference leads to an 0x3B BSOD when anything attempts to use any WMI
method that deals with strings.

Replace UTF8_STRING with OEM_STRING which is available in all WDKs, and has
the same definition as UTF8_STRING from later WDKs, simplifying the usage
of retrieving strings from, or putting strings into, WMI buffers.

Signed-off-by: Owen Smith <owen.smith@citrix.com>