qemu: Refactor tray_status handling

qemu: Refactor tray_status handling

Modify the monitor code to directly return values from 'virDomainDiskTray'
enum instead of two bools.

This allows simplification of the other code:
 - qemuProcessRefreshDiskProps just assigns the value, thus can be deleted
 - old_tray_status is no longer needed as we can directly access the
   disk status instead and assign later

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