Add VM.migration_receive_memory RPC to the xen interface

This call will receive a channel, which will be the one over which
the memory image is streamed.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge branch 'migration-channel' of github.com:djs55/xcp-idl into migration-channel

Conflicts:
	xen/xenops_interface.ml
Move the channel protocol definition into a separate module.

This avoids a recursive module definition problem.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Regenerate OASIS

Signed-off-by: David Scott <dave.scott@eu.citrix.com>