qemu-img: Use error_vreport() in error_exit()

error_exit() uses low-level error_printf() to report errors.
Modernize it to use error_vreport().

Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Max Reitz <mreitz@redhat.com>
Cc: qemu-block@nongnu.org
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20190417190641.26814-2-armbru@redhat.com>