KVM: SVM: Zero out the VMCB array used to track SEV ASID association

Zero out the array of VMCB pointers so that pre_sev_run() won't see
garbage when querying the array to detect when an SEV ASID is being
associated with a new VMCB.  In practice, reading random values is all
but guaranteed to be benign as a false negative (which is extremely
unlikely on its own) can only happen on CPU0 on the first VMRUN and would
only cause KVM to skip the ASID flush.  For anything bad to happen, a
previous instance of KVM would have to exit without flushing the ASID,
_and_ KVM would have to not flush the ASID at any time while building the
new SEV guest.

Cc: Borislav Petkov <bp@suse.de>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Reviewed-by: Brijesh Singh <brijesh.singh@amd.com>
Fixes: 70cd94e6 ("KVM: SVM: VMRUN should use associated ASID when SEV is enabled")
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210422021125.3417167-2-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>