kvm: vfio: silence GCC warning

Building vfio.o triggers a GCC warning (when building for 32 bits x86):
    arch/x86/kvm/../../../virt/kvm/vfio.c: In function 'kvm_vfio_set_group':
    arch/x86/kvm/../../../virt/kvm/vfio.c:104:22: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
      void __user *argp = (void __user *)arg;
                          ^

Silence this warning by casting arg to unsigned long.

argp's current type, "void __user *", is always casted to "int32_t
__user *". So its type might as well be changed to "int32_t __user *".

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>