When I got the "permission denied" error, I found advice to change the user to libvirt-qemu and the group to kvm. Both exist on my system.
$ ls -la
total 15899728
drwxrwxr-x 2 roland roland 4096 Jun 25 16:05 .
drwxr-xr-x 92 roland roland 4096 Jun 25 18:15 ..
-rw------- 1 libvirt-qemu kvm 1474560 Jun 24 20:00 floppy_disk_image
-rw------x 1 libvirt-qemu kvm 1073741824 Jun 24 20:27 WinXPPro-c21a8baf.vmem
-rw------x 1 libvirt-qemu kvm 135507536 Jun 24 20:12 WinXPPro-c21a8baf.vmss
-rw----r-x 1 libvirt-qemu kvm 8684 Jun 24 20:18 WinXPPro.nvram
-rw----r-x 1 libvirt-qemu kvm 2145255424 Jun 24 20:12 WinXPPro-s001.vmdk
-rw----r-x 1 libvirt-qemu kvm 2146238464 Jun 24 20:33 WinXPPro-s002.vmdk
-rw----r-x 1 libvirt-qemu kvm 2146631680 Jun 24 20:25 WinXPPro-s003.vmdk
-rw----r-x 1 libvirt-qemu kvm 3211264 Jun 24 20:12 WinXPPro-s004.vmdk
-rw----r-x 1 libvirt-qemu kvm 2106261504 Jun 24 20:18 WinXPPro-s005.vmdk
-rw----r-x 1 libvirt-qemu kvm 2146697216 Jun 24 20:00 WinXPPro-s006.vmdk
-rw----r-x 1 libvirt-qemu kvm 2162688 Jun 24 20:27 WinXPPro-s007.vmdk
-rw----r-x 1 libvirt-qemu kvm 2146107392 Jun 24 20:06 WinXPPro-s008.vmdk
-rw----r-x 1 libvirt-qemu kvm 2099511296 Jun 24 19:58 WinXPPro-s009.vmdk
-rw-rw-r-x 1 libvirt-qemu kvm 128446464 Jun 24 20:19 WinXPPro-s010.vmdk
-rw----r-x 1 libvirt-qemu kvm 857 Jun 25 15:54 WinXPPro.vmdk
-rw-r--r-x 1 libvirt-qemu kvm 0 Jun 24 20:12 WinXPPro.vmsd
-rwxr-xr-x 1 libvirt-qemu kvm 4209 Jun 25 15:56 WinXPPro.vmx
-rw-r--r-x 1 libvirt-qemu kvm 2932 Jun 25 15:57 WinXPPro.vmxf
-rw-rw-r-x 1 libvirt-qemu kvm 852 Jun 25 16:05 WinXPPro.xml
$ virsh start WinXPPro
error: Failed to start domain WinXPPro
error: internal error process exited while connecting to monitor: qemu-system-i386: -drive file=/home/roland/WinXPPro/WinXPPro.vmdk,if=none,id=drive-ide0-0-0,format=vmdk: could not open disk image /home/roland/WinXPPro/WinXPPro.vmdk: Permission denied
However, despite not using "sudo" and running the command as me, the WinXPPro.vmdk file is now owned by root, so I get the complaint that the file cannot be accessed. If I sudo the command, it gives me the exact same result.
$ ls -la
total 15899728
drwxrwxr-x 2 roland roland 4096 Jun 25 16:05 .
drwxr-xr-x 92 roland roland 4096 Jun 25 18:15 ..
-rw------- 1 libvirt-qemu kvm 1474560 Jun 24 20:00 floppy_disk_image
-rw------x 1 libvirt-qemu kvm 1073741824 Jun 24 20:27 WinXPPro-c21a8baf.vmem
-rw------x 1 libvirt-qemu kvm 135507536 Jun 24 20:12 WinXPPro-c21a8baf.vmss
-rw----r-x 1 libvirt-qemu kvm 8684 Jun 24 20:18 WinXPPro.nvram
-rw----r-x 1 libvirt-qemu kvm 2145255424 Jun 24 20:12 WinXPPro-s001.vmdk
-rw----r-x 1 libvirt-qemu kvm 2146238464 Jun 24 20:33 WinXPPro-s002.vmdk
-rw----r-x 1 libvirt-qemu kvm 2146631680 Jun 24 20:25 WinXPPro-s003.vmdk
-rw----r-x 1 libvirt-qemu kvm 3211264 Jun 24 20:12 WinXPPro-s004.vmdk
-rw----r-x 1 libvirt-qemu kvm 2106261504 Jun 24 20:18 WinXPPro-s005.vmdk
-rw----r-x 1 libvirt-qemu kvm 2146697216 Jun 24 20:00 WinXPPro-s006.vmdk
-rw----r-x 1 libvirt-qemu kvm 2162688 Jun 24 20:27 WinXPPro-s007.vmdk
-rw----r-x 1 libvirt-qemu kvm 2146107392 Jun 24 20:06 WinXPPro-s008.vmdk
-rw----r-x 1 libvirt-qemu kvm 2099511296 Jun 24 19:58 WinXPPro-s009.vmdk
-rw-rw-r-x 1 libvirt-qemu kvm 128446464 Jun 24 20:19 WinXPPro-s010.vmdk
-rw----r-x 1 root root 857 Jun 25 15:54 WinXPPro.vmdk
-rw-r--r-x 1 libvirt-qemu kvm 0 Jun 24 20:12 WinXPPro.vmsd
-rwxr-xr-x 1 libvirt-qemu kvm 4209 Jun 25 15:56 WinXPPro.vmx
-rw-r--r-x 1 libvirt-qemu kvm 2932 Jun 25 15:57 WinXPPro.vmxf
-rw-rw-r-x 1 libvirt-qemu kvm 852 Jun 25 16:05 WinXPPro.xml
roland@Matt-HP:~/WinXPPro$
What is going on here?