x86: adjustments to memory_add()

x86: adjustments to memory_add()

The function should clean up after a failed map_pages_to_xen().

Sharing the M2P table with Dom0 needs to happen before adding the new
pages to the heap (so pages handed out by the allocator will be
represented in what a tool stack may need to map).

Avoid the IOMMU mapping loop whenever possible.

Drop a redundant setting of 'ret'.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>