accel/kvm: check for KVM_CAP_READONLY_MEM on VM

accel/kvm: check for KVM_CAP_READONLY_MEM on VM

KVM_CAP_READONLY_MEM used to be a global capability, but with the
introduction of AMD SEV-SNP confidential VMs, this extension is not
always available on all VM types [1,2].

Query the extension on the VM level instead of on the KVM level.

[1] https://patchwork.kernel.org/project/kvm/patch/20240809190319.1710470-2-seanjc@google.com/
[2] https://patchwork.kernel.org/project/kvm/patch/20240902144219.3716974-1-erbse.13@gmx.de/

Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Tom Dohrmann <erbse.13@gmx.de>
Link: https://lore.kernel.org/r/20240903062953.3926498-1-erbse.13@gmx.de
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
(cherry picked from commit 64e0e63ea16aa0122dc0c41a0679da0ae4616208)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>