xen: enable BALLOON_MEMORY_HOTPLUG by default

Without it a PVH dom0 is mostly useless, as it would balloon down huge
amounts of RAM in order get physical address space to map foreign
memory and grants, ultimately leading to an out of memory situation.

Such option is also needed for HVM or PVH driver domains, since they
also require mapping grants into physical memory regions.

Suggested-by: Ian Jackson <[e-mail]>
Signed-off-by: Roger Pau Monné <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Reviewed-by: Juergen Gross <[e-mail]>
Signed-off-by: Boris Ostrovsky <[e-mail]>

drivers/xen/Kconfig

···
 config XEN_BALLOON_MEMORY_HOTPLUG
 	bool "Memory hotplug support for Xen balloon driver"
 	depends on XEN_BALLOON && MEMORY_HOTPLUG
+	default y
 	help
 	  Memory hotplug support for Xen balloon driver allows expanding memory
 	  available for the system above limit declared at system startup.