bhyve: Escape binary data sent in reply packets

Per https://sourceware.org/gdb/current/onlinedocs/gdb.html/Overview.html#Binary-Data
certain bytes must be escaped. The XML register definitions we have so far do
not run afoul of that rule, but the stub should handle them anyway.

Reviewed by: jhb
MFC after: 2 weeks
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D46194