x86: paravirt_ops: don't steal memory resources in paravirt_disable_iospace

The memory resource is also used for main memory, and we need it to
allocate physical addresses for memory hotplug.  Knobbling io space is
enough to get the job done anyway.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Ingo Molnar <mingo@elte.hu>