kvm: x86: use NULL instead of using plain integer as pointer

Sparse warnings removed:
warning: Using plain integer as NULL pointer

Signed-off-by: Muhammad Usama Anjum <musamaanjum@gmail.com>
Message-Id: <20210305180816.GA488770@LEGION>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>