tools: cleanup domain save switch_qemu_logdirty callback

tools: cleanup domain save switch_qemu_logdirty callback

Move the function into struct save_callbacks with the others and add
the void *closure to the callback arguments.

Add and propagate an error return code from the callback.

Use this in libxl to pass the save context to
libxl__domain_suspend_common_switch_qemu_logdirty allowing us to reuse
the parent's xenstore handle, gc context etc.

Also add an apparently missing libxl__free_all to
libxl__domain_suspend_common.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>