KVM: x86: do not fail if software breakpoint has already been removed

If kvm_arch_remove_sw_breakpoint finds that a software breakpoint does not
have an INT3 instruction, it fails.  This can happen if one sets a
software breakpoint in a kernel module and then reloads it.  gdb then
thinks the breakpoint cannot be deleted and there is no way to add it
back.

Suggested-by: Maxim Levitsky <mlevitsk@redhat.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

target/i386/kvm/kvm.c

@@ -4352,8 +4352,13 @@ int kvm_arch_remove_sw_breakpoint(CPUState *cs, struct kvm_sw_breakpoint *bp)
 {
     uint8_t int3;
 
-    if (cpu_memory_rw_debug(cs, bp->pc, &int3, 1, 0) || int3 != 0xcc ||
-        cpu_memory_rw_debug(cs, bp->pc, (uint8_t *)&bp->saved_insn, 1, 1)) {
+    if (cpu_memory_rw_debug(cs, bp->pc, &int3, 1, 0)) {
+        return -EINVAL;
+    }
+    if (int3 != 0xcc) {
+        return 0;
+    }
+    if (cpu_memory_rw_debug(cs, bp->pc, (uint8_t *)&bp->saved_insn, 1, 1)) {
         return -EINVAL;
     }
     return 0;