vl.c: Replace fprintf(stderr) with error_report()

Straightforward replacement, except for qemu_kill_report(), which
printed a common part of its error message first, then the applicable
special part.  Print each complete message with a single
error_report() instead.

Multi-line messages were replaced by error_report() followed by
error_printf().

The following changes were made to the error messages:

* The "invalid date format" message was reworded to better fit
  the new error_report()+error_printf() pattern.
* On the remaining messages, only the trailing newlines, "qemu:" and
  "error:" message prefixes were removed.
Signed-off-by: NEduardo Habkost <ehabkost@redhat.com>
Message-Id: <1446217682-24421-2-git-send-email-ehabkost@redhat.com>
[Squashed in
Message-Id: <1446217682-24421-3-git-send-email-ehabkost@redhat.com>
and updated commit message]
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>