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/[email protected]/
[2] https://patchwork.kernel.org/project/kvm/patch/[email protected]/

Cc: Paolo Bonzini <[email protected]>
Signed-off-by: Tom Dohrmann <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Paolo Bonzini <[email protected]>
Reviewed-by: Pankaj Gupta <[email protected]>

accel/kvm/kvm-all.c

@@ -2603,7 +2603,7 @@ static int kvm_init(MachineState *ms)
     }
 
     kvm_readonly_mem_allowed =
-        (kvm_check_extension(s, KVM_CAP_READONLY_MEM) > 0);
+        (kvm_vm_check_extension(s, KVM_CAP_READONLY_MEM) > 0);
 
     kvm_resamplefds_allowed =
         (kvm_check_extension(s, KVM_CAP_IRQFD_RESAMPLE) > 0);