bhyveload: add CAP_SEEK to our dirfd rights

In the case of hostbase_fd, this is infact a bug fix; we have a seek
callback that the host: filesystem may use in loader, and we really
don't have a good excuse to break it.

bootfd-derived fds will only be used with fdlopen(3) and rtld doesn't
seem to need pread / lseek at all for it today, but there's no reason to
break if it finds a good reason to later.

Suggested by: markj