If=virtio,boot=on: Invalid parameter 'boot'

I have an AMD 64 system, Kubuntu Natty, custom compiled kernel 2.6.38.8.42, and QEMU version 0.14.1. If I do

$ sudo /usr/local/bin/qemu-system-x86_64 -m 4096 -boot c -drive file=/home/user/Win7.img,if=virtio,boot=on -net nic,model=virtio,vlan=0 -net tap,vlan=0,ifname=tap0,script=/etc/qemu-ifup -localtime -soundhw ac97 -smp 1,cores=6 -vga vmware -usb

I get

qemu-system-x86_64: -drive file=/home/user/Win7.img,if=virtio,boot=on: Invalid parameter ‘boot’

and no boot of my Windows 7 virtual machine. This used to work, and I think it stopped working after the Natty and/or kernel 2.6.38.8.42 upgrades. I’d appreciate ideas as
to how to diagnose and/or resolve the problem. Thanks, Gus

Some other relevant outputs might be

$ ls -l /home/user/Win7.img
-rw-r–r-- 1 user user 46683848704 2011-05-29 07:49 /home/user/Win7.img

$ ifconfig
br0 Link encap:Ethernet HWaddr 00:24:8c:45:c0:b1
inet addr:192.168.1.17 Bcast:192.168.1.255 Mask:255.255.255.0
inet6 addr: fe80::224:8cff:fe45:c0b1/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:19819 errors:0 dropped:0 overruns:0 frame:0
TX packets:20285 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:13151938 (13.1 MB) TX bytes:3483577 (3.4 MB)

eth0 Link encap:Ethernet HWaddr 00:24:8c:45:c0:b1
inet6 addr: fe80::224:8cff:fe45:c0b1/64 Scope:Link
UP BROADCAST RUNNING PROMISC MULTICAST MTU:1500 Metric:1
RX packets:20082 errors:0 dropped:0 overruns:0 frame:0
TX packets:20304 errors:0 dropped:0 overruns:0 carrier:2
collisions:0 txqueuelen:1000
RX bytes:13454250 (13.4 MB) TX bytes:3491557 (3.4 MB)
Interrupt:45

lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
inet6 addr: ::1/128 Scope:Host
UP LOOPBACK RUNNING MTU:16436 Metric:1
RX packets:48 errors:0 dropped:0 overruns:0 frame:0
TX packets:48 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:3006 (3.0 KB) TX bytes:3006 (3.0 KB)

tap0 Link encap:Ethernet HWaddr e2:eb:36:11:40:d0
inet6 addr: fe80::e0eb:36ff:fe11:40d0/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:500
RX bytes:0 (0.0 B) TX bytes:0 (0.0 B)

virbr0 Link encap:Ethernet HWaddr fa:e7:64:48:46:f8
inet addr:192.168.122.1 Bcast:192.168.122.255 Mask:255.255.255.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:97 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:0 (0.0 B) TX bytes:14232 (14.2 KB)

$ lsmod | grep kvm
kvm_amd 49429 0
kvm 337616 1 kvm_amd

lsmod | grep virtio
virtio_balloon 4156 0
virtio_net 12626 0
virtio_blk 5582 0
virtio_console 14259 0
virtio_rng 2195 0
virtio_pci 6477 0
virtio_ring 5535 6 virtio_balloon,virtio_net,virtio_blk,virtio_console,virtio_rng,virtio_pci
virtio 4613 6 virtio_balloon,virtio_net,virtio_blk,virtio_console,virtio_rng,virtio_pci