Add tar container format

Add tar container format

Tar is a very widely used format to store data in. Sometimes people even put
virtual machine images in there.

So it makes sense for qemu to be able to read from tar files. I implemented a
written from scratch reader that also knows about the GNU sparse format, which
is what pigz creates.

This version checks for filenames that end on well-known extensions. The logic
could be changed to search for filenames given on the command line, but that
would require changes to more parts of qemu.

The tar reader in conjunctiuon with dzip gives us the chance to download
tar'ed up virtual machine images (even via http) and instantly make use of
them.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Bruce Rogers <brogers@novell.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
[TH: Use bdrv_open options instead of filename]
Signed-off-by: Tim Hardeck <thardeck@suse.de>
Signed-off-by: Andreas Färber <afaerber@suse.de>