qemuProcessRefreshDisks: Properly compare tray status

qemuProcessRefreshDisks: Properly compare tray status

The code compares the 'tray_open' boolean from 'struct
qemuDomainDiskInfo' directly against 'disk->tray_status' which is
declared as virDomainDiskTray (enum). Now the logic works correctly
because the _OPEN enum has value '1'.

Separate the event emission code from the update code and remember the
old tray state in a separate variable rather than having the sneaky
logic we have today.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>