qemu: Store all iothread's 'poll*' attributes as unsigned long long

qemu: Store all iothread's 'poll*' attributes as unsigned long long

Convert the internal types to unsigned long long. Luckily we can also
covert the external types too:

 - 'qemuDomainSetIOThreadParams' can accept both _UINT and _ULLONG by
   converting to 'virTypedParamsGetUnsigned'

 - querying is handled via the bulk stats API which is flexible:
    - we use virTypedParamListAddUnsigned to use the bigger type only if
      necessary
    - most users don't even notice because the bindings abstract the
      data types

Apart from the code modifications we also improve the documentation
which was missing for the setters.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>