We already have the ability to turn off dumping of guest
RAM via the domain XML. This is not particularly useful
though, as it is under control of the management application.
What is needed is a way for the sysadmin to turn off guest
RAM defaults globally, regardless of whether the mgmt app
provides its own way to set this in the domain XML.

So this adds a 'dump_guest_core' option in /etc/libvirt/qemu.conf
which defaults to false. ie guest RAM will never be included in
the QEMU core dumps by default. This default is different from
historical practice, but is considered to be more suitable as
a default because

 a) guest RAM can be huge and so inflicts a DOS on the host
    I/O subsystem when dumping core for QEMU crashes

 b) guest RAM can contain alot of sensitive data belonging
    to the VM owner. This should not generally be copied
    around inside QEMU core dumps submitted to vendors for
    debugging

 c) guest RAM contents are rarely useful in diagnosing
    QEMU crashes
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>