Add prepare callback to struct VMChangeStateEntry.

The prepare callback is optional and can be set by the new function
qemu_add_vm_change_state_handler_prio_full() that allows setting this
callback in addition to the main callback.

The prepare callbacks and main callbacks are called in two separate
phases: First all prepare callbacks are called and only then all main
callbacks are called.

The purpose of the new prepare callback is to allow all devices to run a
preliminary task before calling the devices' main callbacks.

This will facilitate adding P2P support for VFIO migration where all
VFIO devices need to be put in an intermediate P2P quiescent state
before being stopped or started by the main callback.

Signed-off-by: Avihai Horon <avihaih@nvidia.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Tested-by: YangHang Liu <yanghliu@redhat.com>
Signed-off-by: Cédric Le Goater <clg@redhat.com>