ioemu: Fix e1000 mmio range size.

Per Intel 82540EM Software Developer's Manual pp. 211, the mmio size
is 0x20000, or address overlapping occurs and causes the second card
to fail, which happened to me earlier.

From: Tina Yang <tina.yang@oracle.com>
Signed-off-by: Keir Fraser <keir.fraser@citrix.com>

tools/ioemu/hw/e1000.c

@@ -48,7 +48,7 @@ static int debugflags = DBGBIT(TXERR) | DBGBIT(GENERAL);
 #endif
 
 #define IOPORT_SIZE       0x40
-#define PNPMMIO_SIZE      0x60000
+#define PNPMMIO_SIZE      0x20000
 
 /*
  * HW models: