block: Add support for DictZip enabled gzip files

block: Add support for DictZip enabled gzip files

DictZip is an extension to the gzip format that allows random seeks in gzip
compressed files by cutting the file into pieces and storing the piece offsets
in the "extra" header of the gzip format.

Thanks to that extension, we can use gzip compressed files as block backend,
though only in read mode.

This makes a lot of sense when stacked with tar files that can then be shipped
to VM users. If a VM image is inside a tar file that is inside a DictZip
enabled gzip file, the user can run the tar.gz file as is without having to
extract the image first.

Tar patch follows.

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>
[AF: Error **errp added for bdrv_file_open, bdrv_delete -> bdrv_unref]
[AF: qemu_opts_create_nofail() -> qemu_opts_create(),
     bdrv_file_open() -> bdrv_open(), based on work by brogers]
[AF: error_is_set() dropped for v2.1.0-rc0]
Signed-off-by: Andreas Färber <afaerber@suse.de>