Device{Attach,Detach}: Document S4 limitations

https://bugzilla.redhat.com/show_bug.cgi?id=808463

Well, libvirt doesn't distinguish between domain poweroff and
hibernation (S4). It's hard to differentiate these two on a real
machine anyway. As a result, any device that is hot(un-)plugged is
lost (appears again) when domain is started again as from our POV
it is a fresh cold boot. Instead of doing anything wise here, we
should just document this as known limitation.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>

src/libvirt.c

@@ -10319,6 +10319,10 @@ virNodeGetSecurityModel(virConnectPtr conn, virSecurityModelPtr secmodel)
  * in an existing CDROM/Floppy device, however, applications are
  * recommended to use the virDomainUpdateDeviceFlag method instead.
  *
+ * Be aware that hotplug changes might not persist across a domain going
+ * into S4 state (also known as hibernation) unless you also modify the
+ * persistent domain definition.
+ *
  * Returns 0 in case of success, -1 in case of failure.
  */
 int
@@ -10374,6 +10378,10 @@ virDomainAttachDevice(virDomainPtr domain, const char *xml)
  * in an existing CDROM/Floppy device, however, applications are
  * recommended to use the virDomainUpdateDeviceFlag method instead.
  *
+ * Be aware that hotplug changes might not persist across a domain going
+ * into S4 state (also known as hibernation) unless you also modify the
+ * persistent domain definition.
+ *
  * Returns 0 in case of success, -1 in case of failure.
  */
 int
@@ -10416,6 +10424,10 @@ virDomainAttachDeviceFlags(virDomainPtr domain,
  * Destroy a virtual device attachment to backend.  This function,
  * having hot-unplug semantics, is only allowed on an active domain.
  *
+ * Be aware that hotplug changes might not persist across a domain going
+ * into S4 state (also known as hibernation) unless you also modify the
+ * persistent domain definition.
+ *
  * Returns 0 in case of success, -1 in case of failure.
  */
 int
@@ -10487,6 +10499,10 @@ virDomainDetachDevice(virDomainPtr domain, const char *xml)
  * a synchronous removal. In other words, this API may wait a bit for the
  * removal to complete in case it was not synchronous.
  *
+ * Be aware that hotplug changes might not persist across a domain going
+ * into S4 state (also known as hibernation) unless you also modify the
+ * persistent domain definition.
+ *
  * Returns 0 in case of success, -1 in case of failure.
  */
 int