Enable xen balloon memory hotplug

Allow to not use otherwise usable RAM pages to map foreign
pages (including grant mappings). This is especially useful for dom0 (or
GUI domain) that maps a lot of foreign pages.

QubesOS/qubes-issues#6097

config-qubes

@@ -123,6 +123,12 @@ CONFIG_USB_DUMMY_HCD=m
 
 CONFIG_SECURITY_APPARMOR=y
 
+################################################################################
+## Enable memory hotplug of Xen balloon driver. This is useful to map a lot of
+## grant tables, without using otherwise usable physical address space
+
+CONFIG_XEN_BALLOON_MEMORY_HOTPLUG=y
+CONFIG_XEN_UNPOPULATED_ALLOC=y
 
 ################################################################################
 ## TODO: from diff to old config