Revert "libxl_set_memory_target: retain the same maxmem offset on top of the current...

Revert "libxl_set_memory_target: retain the same maxmem offset on top of the current target"

This reverts commit 0c029c4da2169159064568ef4fea862a5d2cd84a.

A new memory model that allows QEMU to bump memory behind libxl's back
was merged a few months ago. We didn't fully understand the
repercussions back then. Now it breaks migration and becomes blocker of
4.6 release.

It's better to restore to original behaviour at this stage of the
release cycle, that would put us in a position no worse than before, so
the release is unblocked.

The said function is still racy after reverting these two patches.
Making domain memory state consistent requires a bit more work. Separate
patch(es) will be sent out to deal with that problem.

Fix up conflicts with f5b43e95 (libxl: fix "xl mem-set" regression from
0c029c4da2).

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>