vl: Reset location after handling command-line arguments

After looping through all command-line arguments, error location
info becomes obsolete, and any function calling error_report()
will print misleading information. This breaks error reporting
for some option handling, like:

  $ qemu-system-x86_64 -icount rr=x -vnc :0
  qemu-system-x86_64: -vnc :0: Invalid icount rr option: x

  $ qemu-system-x86_64 -m size= -vnc :0
  qemu-system-x86_64: -vnc :0: missing 'size' option value

Fix this by resetting location info as soon as we exit the
command-line handling loop.

With this, replay_configure() and set_memory_options() won't
print any location info yet, but at least they won't print
incorrect information.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <1455303747-19776-3-git-send-email-ehabkost@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
["Do not insert code here" comment added to prevent regressions]
Signed-off-by: Markus Armbruster <armbru@redhat.com>
