qcow2 format: keep 'num_free_bytes', and show it upon 'info blockstats' (Uri Lublin)

'num_free_bytes' is the number of non-allocated bytes below highest-allocation.
It's useful, together with the highest-allocation, to figure out how
fragmented the image is, and how likely it will run out-of-space soon.

For example when the highest allocation is high (almost end-of-disk), but 
many bytes (clusters) are free, and can be re-allocated when neeeded, than
we know it's probably not going to reach end-of-disk-space soon.

Added bookkeeping to block-qcow2.c
Export it using BlockDeviceInfo
Show it upon 'info blockstats' if BlockDeviceInfo exists

Signed-off-by: Uri Lublin <uril@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>


git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6407 c046a42c-6fe2-441c-8c8c-71466251a162

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