qemu: Add flag to force a CDROM eject

QEMU allows forcing a CDROM eject even if the guest has locked the device.
Expose this via a new UpdateDevice flag, VIR_DOMAIN_DEVICE_MODIFY_FORCE.

This has been requested for RHEV:

https://bugzilla.redhat.com/show_bug.cgi?id=626305

v2: Change flag name, bool cleanups

include/libvirt/libvirt.h.in

@@ -1033,6 +1033,8 @@ typedef enum {
    VIR_DOMAIN_DEVICE_MODIFY_CURRENT = 0, /* Modify device allocation based on current domain state */
    VIR_DOMAIN_DEVICE_MODIFY_LIVE = (1 << 0), /* Modify live device allocation */
    VIR_DOMAIN_DEVICE_MODIFY_CONFIG = (1 << 1), /* Modify persisted device allocation */
+   VIR_DOMAIN_DEVICE_MODIFY_FORCE = (1 << 2), /* Forcibly modify device
+                                                 (ex. force eject a cdrom) */
 } virDomainDeviceModifyFlags;
 
 int virDomainAttachDevice(virDomainPtr domain, const char *xml);