tools/libxc: Disable CONFIG_MIGRATE in stubdom environments

tools/libxc: Disable CONFIG_MIGRATE in stubdom environments

The legacy save/restore infrastructure requires several function pointers from
the toolstack (libxl or Xend in the past) in order to work, and for HVM guests
also need to be able to play around in dom0's filesystem to move the device
model save record.

Migration v2 changes some of this, but is similarly dependent on
toolstack-provided function pointers.

Someone who wishes to re-architect the interaction of moving parts for running
a domain might be in a position to re-enabled this, but for now, explicitly
fail with ENOSYS (from xc_nomigrate.c) rather than failing with an error about
a missing function pointer (or indeed falling over a NULL pointer on certain
paths).

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
CC: Ian Campbell <Ian.Campbell@citrix.com>
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>