qemu: enable QEMU core dump by default on Linux

qemu: enable QEMU core dump by default on Linux

The Linux MADV_DONTDUMP flag was added to Linux kernels > 3.3,
back in 2012, and the dump-guest-core flag was added to QEMU
> 1.0 at the same time.

IOW, on Linux we have long been able to assume that QEMU core
dumps will exclude guest memory, unless the user has overridden
the host level defaults in the domain XML.

It is desirable to permit QEMU core dumps out of the box to make
it easier for users to report crashes to their OS vendor without
having to reconfigure and restart libvirt daemons and their
running guests.

While there is a risk that an admin may have set 'dump_guest_core'
to true, while leaving 'max_core' to 0, on balance the benefits
of easier troubleshooting outweigh the risk of changing the
defaults to permit core dumps.

Reviewed-by: Richard W.M. Jones <rjones@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>