MFC r352925: xen/ctrl: acknowledge all control requests

MFC r352925: xen/ctrl: acknowledge all control requests
MFC r357616: xen/console: fix priority of Xen console
MFC r361274: dev/xenstore: fix return with locks held
Note this should be dev/evtchn not dev/xenstore.
MFC r361578: xenpv: do not use low 1MB for Xen mappings on i386
MFC r361580: xen/control: short circuit xctrl_on_watch_event on spurious event

Those are all Xen related fixes or minor improvements that have been sitting on
current for a reasonable time without complaints.

Sponsored by: Citrix Systems R&D