SailBugfix: scounteren is read-only except for IR, TMP, and CY fields

This commit makes all bits in the mcounteren register read-only, except for the aforementioned IR, TMP, and CY fields, which remain writable.

src/virt/csr.rs

@@ -520,7 +520,10 @@ impl HwRegisterContextSetter<Csr> for VirtContext {
                     }
                 }
             }
-            Csr::Scounteren => (), // Read-only 0
+            Csr::Scounteren => {
+                // Only show IR, TM and CY (for cycle, time and instret counters)
+                self.csr.scounteren = (self.csr.scounteren & !0b111) | (value & 0b111) as u32
+            }
             Csr::Senvcfg => self.csr.senvcfg = value,
             Csr::Sscratch => self.csr.sscratch = value,
             Csr::Sepc => {