Use qemu-img -o preallocation=metadata where possible.

When format is qcow2 and we're *not* using a backing disk, we can use
preallocation=metadata for better performance.
Add man page for /etc/libguestfs-tools.conf (RHBZ#1019889).

Add a man page to document this configuration file.  This is
required by some distros, eg. Debian and RHEL.