migration: push Error **errp into qemu_loadvm_section_part_end()

migration: push Error **errp into qemu_loadvm_section_part_end()

This is an incremental step in converting vmstate loading
code to report error via Error objects instead of directly
printing it to console/monitor.
It is ensured that qemu_loadvm_section_part_end() must report an error
in errp, in case of failure.
This patch also removes the setting of errp when errp is NULL in the
out section as it is no longer required in the series.

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Signed-off-by: Arun Menon <armenon@redhat.com>
Tested-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Akihiko Odaki <odaki@rsg.ci.i.u-tokyo.ac.jp>
Link: https://lore.kernel.org/r/20250918-propagate_tpm_error-v14-11-36f11a6fb9d3@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>