qemu.py: use poll() instead of 'returncode'

The 'returncode' Popen attribute is not guaranteed to be updated. It
actually depends on a call to either poll(), wait() or communicate().

On the other hand, poll() will: "Check if child process has terminated.
Set and return returncode attribute."

Let's use the poll() to check whether the process is running and to get
the updated process exit code, when the process is finished.

Reviewed-by: Fam Zheng <famz@redhat.com>
eviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Amador Pahim <apahim@redhat.com>
Message-Id: <20180122205033.24893-5-apahim@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>

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