libxenguest: add get_unaligned_le32()

libxenguest: add get_unaligned_le32()

Abstract xc_dom_check_gzip()'s reading of the uncompressed size into a
helper re-usable, in particular, by other decompressor code.

Sadly in the mini-os case this conflicts with other functions of the
same name (and purpose), which can't be easily replaced individually.
Yet it was requested that no full set of helpers be introduced at this
point in the release cycle. Hence the awkward XG_NEED_UNALIGNED.

Requested-by: Ian Jackson <iwj@xenproject.org>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Ian Jackson <ian.jackson@eu.citrix.com>
Release-Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>