tools/migrate: Fix regression when migrating from older version of Xen

tools/migrate: Fix regression when migrating from older version of Xen

Commit 00a4b65f8534c9e6521eab2e6ce796ae36037774 Sep 7 2010
  "libxc: provide notification of final checkpoint to restore end"
broke migration from any version of Xen using tools from prior to that commit

Older tools have no idea about an XC_SAVE_ID_LAST_CHECKPOINT, causing newer
tools xc_domain_restore() to start reading the qemu save record, as
ctx->last_checkpoint is 0.

The failure looks like:
  xc: error: Max batch size exceeded (1970103633). Giving up.
where 1970103633 = 0x756d6551 = *(uint32_t*)"Qemu"

With this fix in place, the behaviour for normal migrations is reverted to how
it was before the regression; the migration is considered non-checkpointed
right from the start.  A XC_SAVE_ID_LAST_CHECKPOINT chunk seen in the
migration stream is a nop.  For checkpointed migrations the behaviour is
unchanged.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Campbell <Ian.Campbell@citrix.com>
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Shriram Rajagopalan <rshriram@cs.ubc.ca> (Remus bits)