KVM: SVM: Get source vCPUs from source VM for SEV-ES intrahost migration

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

KVM: SVM: Get source vCPUs from source VM for SEV-ES intrahost migration

Fix a goof where KVM tries to grab source vCPUs from the destination VM
when doing intrahost migration. Grabbing the wrong vCPU not only hoses
the guest, it also crashes the host due to the VMSA pointer being left
NULL.

BUG: unable to handle page fault for address: ffffe38687000000
#PF: supervisor read access in kernel mode
#PF: error_code(0x0000) - not-present page
PGD 0 P4D 0
Oops: 0000 [#1] SMP NOPTI
CPU: 39 PID: 17143 Comm: sev_migrate_tes Tainted: GO 6.5.0-smp--fff2e47e6c3b-next #151
Hardware name: Google, Inc. Arcadia_IT_80/Arcadia_IT_80, BIOS 34.28.0 07/10/2023
RIP: 0010:__free_pages+0x15/0xd0
RSP: 0018:ffff923fcf6e3c78 EFLAGS: 00010246
RAX: 0000000000000000 RBX: ffffe38687000000 RCX: 0000000000000100
RDX: 0000000000000100 RSI: 0000000000000000 RDI: ffffe38687000000
RBP: ffff923fcf6e3c88 R08: ffff923fcafb0000 R09: 0000000000000000
R10: 0000000000000000 R11: ffffffff83619b90 R12: ffff923fa9540000
R13: 0000000000080007 R14: ffff923f6d35d000 R15: 0000000000000000
FS: 0000000000000000(0000) GS:ffff929d0d7c0000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: ffffe38687000000 CR3: 0000005224c34005 CR4: 0000000000770ee0
PKRU: 55555554
Call Trace:
<TASK>
sev_free_vcpu+0xcb/0x110 [kvm_amd]
svm_vcpu_free+0x75/0xf0 [kvm_amd]
kvm_arch_vcpu_destroy+0x36/0x140 [kvm]
kvm_destroy_vcpus+0x67/0x100 [kvm]
kvm_arch_destroy_vm+0x161/0x1d0 [kvm]
kvm_put_kvm+0x276/0x560 [kvm]
kvm_vm_release+0x25/0x30 [kvm]
__fput+0x106/0x280
____fput+0x12/0x20
task_work_run+0x86/0xb0
do_exit+0x2e3/0x9c0
do_group_exit+0xb1/0xc0
__x64_sys_exit_group+0x1b/0x20
do_syscall_64+0x41/0x90
entry_SYSCALL_64_after_hwframe+0x63/0xcd
</TASK>
CR2: ffffe38687000000