vl: correct error message when fail to init kvm

command:
qemu-system-x86_64 -hda disk.img -smp 32 --enable-kvm
error:
Number of SMP cpus requested (32) exceeds max cpus supported by KVM (16)
failed to initialize KVM: Invalid argument
No accelerator found!

well, it did find kvm, but failed to init,
so message "No accelerator found!" is confusing,
this commit remove the confusing error message.

Signed-off-by: liguang <lig.fnst@cn.fujitsu.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
