KVM: SEV: Lock all vCPUs when synchronzing VMSAs for SNP launch finish

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

KVM: SEV: Lock all vCPUs when synchronzing VMSAs for SNP launch finish

Lock all vCPUs when synchronizing and encrypting VMSAs for SNP guests, as

allowing userspace to manipulate and/or run a vCPU while its state is being

synchronized would at best corrupt vCPU state, and at worst crash the host

kernel.

Opportunistically assert that vcpu->mutex is held when synchronizing its

VMSA (the SEV-ES path already locks vCPUs).