Fix build with libc++ 19

[rbgarga]

As noted in the libc++ 19 release notes [1], std::char_traits<> is now only provided for char, char8_t, char16_t, char32_t and wchar_t, and any instantiation for other types will fail.

This causes emulators/open-vm-tools to fail to compile with clang 19 and libc++ 19.

This can be fixed by using the standard type std::u16string for UTF-16 strings, instead of (effectively) std::basic_string<uint16_t>.

[1] https://libcxx.llvm.org/ReleaseNotes/19.html#deprecations-and-removals