dump: do not dump non-existent guest memory

dump: do not dump non-existent guest memory

It does not really make sense to dump memory that is not there.

Moreover, that fixes a segmentation fault when calling dump-guest-memory
with no filter for a machine with no memory defined.

New behaviour is:

(qemu) dump-guest-memory /dev/null
dump: no guest memory to dump
(qemu) dump-guest-memory /dev/null 0 4096
dump: no guest memory to dump

Signed-off-by: Cornelia Huck <cohuck@redhat.com>
Tested-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Peter Xu <peterx@redhat.com>
Message-Id: <20170913142036.2469-4-lvivier@redhat.com>
Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>