KVM: SEV: Fall back to vmalloc for SEV-ES scratch area if necessary

Use kvzalloc() to allocate KVM's buffer for SEV-ES's GHCB scratch area so
that KVM falls back to __vmalloc() if physically contiguous memory isn't
available.  The buffer is purely a KVM software construct, i.e. there's
no need for it to be physically contiguous.

Cc: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20211109222350.2266045-3-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>