qtest: unlink QEMU pid file after startup

After starting the QEMU process and initializing the QMP connection, we
can read the pid file and unlink it.

Just stash away the pid instead of the pid filename.  This way we can
avoid pid file leaks since running tests may abort(3) without cleanup.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
qtest: unlink UNIX domain sockets after connecting

UNIX domain sockets are leaked when tests call abort(3) (indirectly via
glib assert functions).

Unlink the files immediately after the connection has been established
to avoid leaks.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Merge remote-tracking branch 'remotes/stefanha/tags/qtest-for-peter' into staging

qtest resource cleanup patches

# gpg: Signature made Tue 04 Feb 2014 08:29:12 GMT using RSA key ID 81AB73C8
# gpg: Good signature from "Stefan Hajnoczi <stefanha@redhat.com>"
# gpg:                 aka "Stefan Hajnoczi <stefanha@gmail.com>"
# gpg: WARNING: This key is not certified with a trusted signature!
# gpg:          There is no indication that the signature belongs to the owner.
# Primary key fingerprint: 8695 A8BF D3F9 7CDA AC35  775A 9CA4 ABB3 81AB 73C8

* remotes/stefanha/tags/qtest-for-peter:
  qtest: unlink UNIX domain sockets after connecting
  qtest: unlink QEMU pid file after startup

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>