tests: Avoid 'do/while(false); ' in vhost-user-bridge

Use of a do/while(0) loop as a way to allow break statements in
the middle of execute-once code is unusual.  More typical is
the use of goto for early exits, with a label at the end of
the execute-once code, rather than nesting code in a scope;
however, the comment at the end of the existing code makes this
alternative a bit unpractical.

So, to avoid false positives from a future syntax check about
'while (false);', and to keep the loop form (in case someone
ever does add DONTWAIT support, where they can just as easily
manipulate the initial loop condition or add an if around the
final 'break'), I opted to use the form of a while(1) loop (the
break as an early exit is more idiomatic there), coupled with
a final break preserving the original comment.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20171201232433.25193-6-eblake@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
