common/Formatter: fix string_view usage for {json,xml}_stream_escaper

common/Formatter: fix string_view usage for {json,xml}_stream_escaper

These are passing a simple const char* without a length, which breaks for
binary strings with \0 in them.

Broken by b40f76a7a0cf460f2a851b001885a4831a6f6503.

This code is still a mess because there is a mix of boost::string_view
and std::string_view throughout the code.  A cleanup is in order!

Signed-off-by: Sage Weil <sage@redhat.com>