KVM: x86: Only advertise KVM_CAP_READONLY_MEM when supported by VM

Until recently, KVM_CAP_READONLY_MEM was unconditionally supported on
x86, but this is no longer the case for SEV-ES and SEV-SNP VMs.

When KVM_CHECK_EXTENSION is invoked on a VM, only advertise
KVM_CAP_READONLY_MEM when it's actually supported.

Fixes: 66155de9 ("KVM: x86: Disallow read-only memslots for SEV-ES and SEV-SNP (and TDX)")
Cc: Sean Christopherson <seanjc@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Michael Roth <michael.roth@amd.com>
Signed-off-by: Tom Dohrmann <erbse.13@gmx.de>
Message-ID: <20240902144219.3716974-1-erbse.13@gmx.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>