migration: Remove error variant of vmstate_save_state() function

migration: Remove error variant of vmstate_save_state() function

This commit removes the redundant vmstate_save_state_with_err()
function.

Previously, commit 969298f9d7 introduced vmstate_save_state_with_err()
to handle error propagation, while vmstate_save_state() existed for
non-error scenarios.
This is because there were code paths where vmstate_save_state_v()
(called internally by vmstate_save_state) did not explicitly set
errors on failure.

This change unifies error handling by
 - updating vmstate_save_state() to accept an Error **errp argument.
 - vmstate_save_state_v() ensures errors are set directly within the errp
   object, eliminating the need for two separate functions.

All calls to vmstate_save_state_with_err() are replaced with
vmstate_save_state(). This simplifies the API and improves code
maintainability.

vmstate_save_state() that only calls vmstate_save_state_v(),
by inference, also has errors set in errp in case of failure.
The errors are reported using error_report_err().
If we want the function to exit on error, then &error_fatal is
passed.

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-v14-24-36f11a6fb9d3@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>