qemu: Properly avoid cancelling memory-only dump

qemu: Properly avoid cancelling memory-only dump

migrate_cancel QMP command cannot be used for cancelling memory-only
dumps and priv->job.dump_memory_only is used for reporting an error if
someone calls virDomainAbortJob when memory-only dump job is running.

Since commit 150930e3098 the dump_memory_only flag is set only if
dump-guest-memory command was called without the detach parameter. This
would incorrectly allow libvirt to send migrate_cancel while the
detached memory-only dump is running.

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>