KVM: remove long -> void *user -> long cast

kvm_dev_ioctl casts the arg value to void __user *, just to recast it
again to long. This seems unnecessary.

According to objdump the binary code on x86 is unchanged by this patch.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>