qdev: Introduce lost tick policy property

Potentially tick-generating timer devices will gain a common property:
lock_tick_policy. It allows to encode 4 different ways how to deal with
tick events the guest did not process in time:

discard - ignore lost ticks (e.g. if the guest compensates for them
          already)
delay   - replay all lost ticks in a row once the guest accepts them
          again
merge   - if multiple ticks are lost, all of them are merged into one
          which is replayed once the guest accepts it again
slew    - lost ticks are gradually replayed at a higher frequency than
          the original tick

Not all timer device will need to support all modes. However, all need
to accept the configuration via this common property.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

qemu-common.h

@@ -250,6 +250,13 @@ typedef struct QEMUSGList QEMUSGList;
 
 typedef uint64_t pcibus_t;
 
+typedef enum LostTickPolicy {
+    LOST_TICK_DISCARD,
+    LOST_TICK_DELAY,
+    LOST_TICK_MERGE,
+    LOST_TICK_SLEW,
+} LostTickPolicy;
+
 void tcg_exec_init(unsigned long tb_size);
 bool tcg_enabled(void);