block/qapi: Fix Sparse warning

Sparse reports this warning:

block/qapi.c:417:47: warning:
 too long initializer-string for array of char(no space for nul char)

Replacing the string by an array of characters fixes this warning.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
