KVM: use vma_pages() helper

Use vma_pages function on vma object instead of explicit computation.

Fix the following coccicheck warning:
./virt/kvm/kvm_main.c:3526:29-35: WARNING: Consider using vma_pages
helper on vma

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Message-Id: <1632900526-119643-1-git-send-email-yang.lee@linux.alibaba.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>