qga: Rewrite code where using readdir_r

If readdir_r fails, error_setg_errno will reference the freed
pointer *dirpath*.

Moreover, readdir_r may cause a buffer overflow, using readdir instead.

Signed-off-by: zhanghailiang <zhang.zhanghailiang@huawei.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Merge remote-tracking branch 'remotes/mdroth/tags/qga-pull-2014-10-22-tag' into staging

qga: remove readdir_r usage and fix use-after-free

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

# gpg: Signature made Wed 22 Oct 2014 13:56:19 BST using RSA key ID F108B584
# gpg: Can't check signature: public key not found

* remotes/mdroth/tags/qga-pull-2014-10-22-tag:
  qga: Rewrite code where using readdir_r

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