libxl: qmp: ensure qmp read buffer is NULL terminated

libxl: qmp: ensure qmp read buffer is NULL terminated

Coverity rightly points out that qmp->buffer may not be NULL
terminated when passed to strncat.

Make the actual buffer a byte bigger than QMP_RECEIVE_BUFFER_SIZE and
always append a NULL byte.

I suspect that in practice we have not yet seen QMP messages
approaching the buffer size (4K).

Compile tested only.

CID: 1055989

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>