q35: make pci window address/size match guest cfg

For Q35, MMCFG address and size are guest configurable.
Update w32 property to make it behave accordingly.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/pci-host/q35.c

@@ -214,6 +214,16 @@ static void mch_update_pciexbar(MCHPCIState *mch)
     }
     addr = pciexbar & addr_mask;
     pcie_host_mmcfg_update(pehb, enable, addr, length);
+    /* Leave enough space for the MCFG BAR */
+    /*
+     * TODO: this matches current bios behaviour, but it's not a power of two,
+     * which means an MTRR can't cover it exactly.
+     */
+    if (enable) {
+        mch->pci_info.w32.begin = addr + length;
+    } else {
+        mch->pci_info.w32.begin = MCH_HOST_BRIDGE_PCIEXBAR_DEFAULT;
+    }
 }
 
 /* PAM */