migration: push Error **errp into qemu_loadvm_state_main()

migration: push Error **errp into qemu_loadvm_state_main()

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_state_main() must report an error
in errp, in case of failure.

Set errp explicitly if it is NULL in case of failure in the out
section. This will be removed in the subsequent patch when all of
the calls are converted to passing errp.

The error message in the default case of qemu_loadvm_state_main()
has the word "savevm". This is removed because it can confuse the
user while reading destination side error logs.

Reviewed-by: Daniel P. Berrangé <berrange@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-v14-9-36f11a6fb9d3@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>