qapi: Drop support for escape sequences other than \\

qapi: Drop support for escape sequences other than \\

Since the previous commit restricted strings to printable ASCII,
\uXXXX's only use is obfuscation.  Drop it.

This leaves \\, \/, \', and \".  Since QAPI schema strings are all
names, and names are restricted to ASCII letters, digits, hyphen, and
underscore, none of them is useful.

The latter three have no test coverage.  Drop them.

Keep \\ to avoid (more) gratuitous incompatibility with JSON.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20190913201349.24332-8-armbru@redhat.com>