qxl: allowing the command rings to be not empty when spice worker is stopped RHBZ #728984

same as 8927cfbba232e28304734f7afd463c1b84134031, but for qxl_check_state, that was
triggered by qxl_pre_load (which calls qxl_hard_reset, which calls qxl_soft_reset),
and caused the migration target to crash.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
