block/iscsi: query for supported VPD pages

this patch ensures that we only query for block provisioning and
block limits vpd pages if they are advertised. It also cleans
up the inquiry code and eliminates some redundant code.

Signed-off-by: Peter Lieven <pl@kamp.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
