KVM: VMX: Do _all_ initialization before exposing /dev/kvm to userspace

In the Linux kernel, the following vulnerability has been resolved:

KVM: VMX: Do _all_ initialization before exposing /dev/kvm to userspace

Call kvm_init() only after _all_ setup is complete, as kvm_init() exposes
/dev/kvm to userspace and thus allows userspace to create VMs (and call
other ioctls). E.g. KVM will encounter a NULL pointer when attempting to
add a vCPU to the per-CPU loaded_vmcss_on_cpu list if userspace is able to
create a VM before vmx_init() configures said list.

BUG: kernel NULL pointer dereference, address: 0000000000000008
#PF: supervisor write access in kernel mode
#PF: error_code(0x0002) - not-present page
PGD 0 P4D 0
Oops: 0002 [#1] SMP
CPU: 6 PID: 1143 Comm: stable Not tainted 6.0.0-rc7+ #988
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
RIP: 0010:vmx_vcpu_load_vmcs+0x68/0x230 [kvm_intel]
<TASK>
vmx_vcpu_load+0x16/0x60 [kvm_intel]
kvm_arch_vcpu_load+0x32/0x1f0 [kvm]
vcpu_load+0x2f/0x40 [kvm]
kvm_arch_vcpu_create+0x231/0x310 [kvm]
kvm_vm_ioctl+0x79f/0xe10 [kvm]
? handle_mm_fault+0xb1/0x220
__x64_sys_ioctl+0x80/0xb0
do_syscall_64+0x2b/0x50
entry_SYSCALL_64_after_hwframe+0x46/0xb0
RIP: 0033:0x7f5a6b05743b
</TASK>
Modules linked in: vhost_net vhost vhost_iotlb tap kvm_intel(+) kvm irqbypass