Fix: libcrmcommon: Escape XML non-printing characters correctly as hex

Fixes regression introduced in c6d9cea9.

That commit introduced pcmk__xml_escape(), which (among other things)
escapes non-printing characters as their hexadecimal character
references. However, a prefix of "#x" (instead of only "#") is required
to indicate that the number is in hex rather than decimal.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>