intel_iommu: fix operator in vtd_switch_address_space

When calculating use_iommu, we wanted to first detect whether DMAR is
enabled, then check whether PT is enabled if DMAR is enabled.  However
in the current code we used "&" rather than "&&" so the ordering
requirement is lost (instead it'll be an "AND" operation).  This could
introduce errors dumped in QEMU console when rebooting a guest with
both assigned device and vIOMMU, like:

  qemu-system-x86_64: vtd_dev_to_context_entry: invalid root entry:
  rsvd=0xf000ff53f000e2c3, val=0xf000ff53f000ff53 (reserved nonzero)
Acked-by: NJason Wang <jasowang@redhat.com>
Signed-off-by: NPeter Xu <peterx@redhat.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

hw/i386/intel_iommu.c

@@ -1153,7 +1153,7 @@ static bool vtd_switch_address_space(VTDAddressSpace *as)
 
     assert(as);
 
-    use_iommu = as->iommu_state->dmar_enabled & !vtd_dev_pt_enabled(as);
+    use_iommu = as->iommu_state->dmar_enabled && !vtd_dev_pt_enabled(as);
 
     trace_vtd_switch_address_space(pci_bus_num(as->bus),
                                    VTD_PCI_SLOT(as->devfn),