launch: Close file descriptors after fork (RHBZ#1123007).

This refactors existing code to close file descriptors in the recovery
process, and also adds code to close file descriptors between the
fork() and exec() of QEMU or User-Mode Linux.

The reason is to avoid leaking main process file descriptors where the
main process (or other libraries in the main process) are not setting
O_CLOEXEC at all or not setting it atomically.  Python is a particular
culprit.

See also this OpenStack Nova bug report:
https://bugs.launchpad.net/nova/+bug/1313477

Thanks: Qin Zhao for identifying and characterizing the problem in Nova.
Add qemu-speed-test program to test speed of qemu.

Currently it tests the upload and download speed of virtio-serial and
the read and write speed of the block device (eg. virtio-scsi).