qemu: Fix build error

People seeing this:

   $ go get github.com/cloudius-systems/capstan
   $ git clone https://github.com/syuu1228/capstan-mruby-simplehttpserver.git
   $ cd cpastan-mruby-simplehttpserver
   $ capstan run
   Building capstan-mruby-simplehttpserver...
   Fetching cloudius/osv-base/index.yaml...134 bytes downloaded.
   Fetching cloudius/osv-base/osv-base.qemu.gz...16384000 bytes downloaded.
   dial tcp 127.0.0.1:10000: connection refused

This is a time issue between we start the vm and connect to the vm.
Use util.ConnectAndWait() to wait and try more.

Signed-off-by: Asias He <asias@cloudius-systems.com>