libxc: don't fail domain creation when unpacking initrd fails

libxc: don't fail domain creation when unpacking initrd fails

At least Linux kernels have been able to work with gzip-ed initrd for
quite some time; initrd compressed with other methods aren't even being
attempted to unpack. Furthermore the unzip-ing routine used here isn't
capable of dealing with various forms of concatenated files, each of
which was gzip-ed separately (it is this particular case which has been
the source of observed VM creation failures).

Hence, if unpacking fails, simply hand the compressed blob to the guest
as is.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>