conf: limit chassisNr, and busNr to a minimum value of 1, not 0

In the case of chassisNr (used to set chassis_nr of a pci-bridge
controller), 0 is reserved for / used by the pci[e]-root bus. In the
base of busNr, a value of 0 would mean that the root bus had no places
available to plug in new buses, including the pxb itself (the
documentation I wrote for pxb even noted the limit of busNr as 1.254).

NB: oddly, the "chassis" attribute, which is used for pcie-root-port
and pcie-switch-downstream-port *can* be set to 0, since it's the
combination of {chassis, slot} that needs to be unique, not chassis by
itself (and slot 0 of pcie-root is reserved, while pcie-*-port can use
*only* slot 0).

This resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1342962

docs/formatdomain.html.in

@@ -3277,7 +3277,7 @@
         control QEMU's "chassis_nr" option for the pci-bridge device
         (normally libvirt automatically sets this to the same value as
         the index attribute of the pci controller). If set, chassisNr
-        must be between 0 and 255.
+        must be between 1 and 255.
       </dd>
       <dt><code>chassis</code></dt>
       <dd>