KVM: SVM: Use kzalloc for sev ioctl interfaces to prevent kernel data leak

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

KVM: SVM: Use kzalloc for sev ioctl interfaces to prevent kernel data leak

For some sev ioctl interfaces, the length parameter that is passed maybe

less than or equal to SEV_FW_BLOB_MAX_SIZE, but larger than the data

that PSP firmware returns. In this case, kmalloc will allocate memory

that is the size of the input rather than the size of the data.

Since PSP firmware doesn't fully overwrite the allocated buffer, these

sev ioctl interface may return uninitialized kernel slab memory.