qemu: avoid denial of service reading from QEMU monitor (CVE-2018-5748)

qemu: avoid denial of service reading from QEMU monitor (CVE-2018-5748)

We read from QEMU until seeing a \r\n pair to indicate a completed reply
or event. To avoid memory denial-of-service though, we must have a size
limit on amount of data we buffer. 10 MB is large enough that it ought
to cope with normal QEMU replies, and small enough that we're not
consuming unreasonable mem.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>