qemu: Delete thread-context objects at domain startup

qemu: Delete thread-context objects at domain startup

While technically thread-context objects can be reused, we only
use them (well, will use them) to pin memory allocation threads.
Therefore, once we connect to QEMU monitor, all memory (with
prealloc=yes) was allocated and thus these objects are no longer
needed and can be removed. For on demand allocation the TC object
is left behind.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>