postcopy: Allow shared memory

Now that we have the mechanisms in here, allow shared memory in a
postcopy.

Note that QEMU can't tell who all the users of shared regions are
and thus can't tell whether all the users of the shared regions
have appropriate support for postcopy.  Those devices that explicitly
support shared memory (e.g. vhost-user) must check, but it doesn't
stop weirder configurations causing problems.
Signed-off-by: NDr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

migration/postcopy-ram.c

@@ -215,12 +215,6 @@ static int test_ramblock_postcopiable(const char *block_name, void *host_addr,
     RAMBlock *rb = qemu_ram_block_by_name(block_name);
     size_t pagesize = qemu_ram_pagesize(rb);
 
-    if (qemu_ram_is_shared(rb)) {
-        error_report("Postcopy on shared RAM (%s) is not yet supported",
-                     block_name);
-        return 1;
-    }
-
     if (length % pagesize) {
         error_report("Postcopy requires RAM blocks to be a page size multiple,"
                      " block %s is 0x" RAM_ADDR_FMT " bytes with a "