chardev: Use goto/label instead of do/break/while(0)

Use of a do/while(0) control flow in order to permit an early break
is an unusual paradigm, and triggers a false positive with a planned
future syntax check against 'while (0);'.  Rewrite the code to use a
goto instead.  This patch temporarily keeps an extra level of
indentation to highlight the change; the next patch cleans it up.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20171201232433.25193-4-eblake@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
