tools/hv: reduce resource usage in hv_kvp_daemon

tools/hv: reduce resource usage in hv_kvp_daemon

hv_kvp_daemon uses popen(3) and system(3) as convinience helper to
launch external helpers. These helpers are invoked via a
temporary shell process. There is no need to keep this temporary
process around while the helper runs. Replace this temporary shell
with the actual helper process via 'exec'.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Link: https://lore.kernel.org/linux-hyperv/20241202123520.27812-1-olaf@aepfle.de/
Signed-off-by: Wei Liu <wei.liu@kernel.org>