qga: Restrict guest-file-read count to 48 MB to avoid crashes

On [*] Daniel Berrangé commented:

  The QEMU guest agent protocol is not sensible way to access huge
  files inside the guest. It requires the inefficient process of
  reading the entire data into memory than duplicating it again in
  base64 format, and then copying it again in the JSON serializer /
  monitor code.

  For arbitrary general purpose file access, especially for large
  files, use a real file transfer program or use a network block
  device, not the QEMU guest agent.

To avoid bug reports as BZ#1594054 (CVE-2018-12617), follow his
suggestion to put a low, hard limit on "count" in the guest agent
QAPI schema, and don't allow count to be larger than 48 MB.

[*] https://www.mail-archive.com/qemu-devel@nongnu.org/msg693176.html

Fixes: CVE-2018-12617
Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1594054


Reported-by: Fakhri Zulkifli <mohdfakhrizulkifli@gmail.com>
Suggested-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
*update schema documentation to indicate 48MB limit instead of 10MB
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>