migration: Fix !replay_can_snapshot() error handling

migration: Fix !replay_can_snapshot() error handling

Calling error_report() in a function that takes an Error ** argument
is suspicious.  save_snapshot() and load_snapshot() do that, and then
fail without setting an error.  Wrong.  The HMP commands survive this
unscathed, since hmp_handle_error() does nothing when no error has
been set.  Callers main() (on behalf of -loadvm) and
replay_vmstate_init() crash, but I'm not sure the error is possible
there.

Screwed up when commit 377b21ccea1 (v2.12.0) added incorrect error
handling right next to correct examples.  Fix by calling error_setg()
instead of error_report().

Fixes: 377b21ccea1755a8b0dae822c29567c58dda6939
Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20181017082702.5581-13-armbru@redhat.com>