None of the fields actually return negative values. The internal
implementation of BlockAcctStats struct in qemu uses uint64_t and the
last place using -1 in libvirt was in the HMP monitor code which was
deleted.

Change the internal type to unsigned long long and ensure that all
public conversions don't overflow.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>