option: Fix checking of sizes for overflow and trailing crap

parse_option_size()'s checking for overflow and trailing crap is
wrong.  Has always been that way.  qemu_strtosz() gets it right, so
use that.

This adds support for size suffixes 'P', 'E', and ignores case for all
suffixes, not just 'k'.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <1487708048-2131-25-git-send-email-armbru@redhat.com>
