x86/shim: fix ballooning down the guest

x86/shim: fix ballooning down the guest

Currently ballooning down a pvshim guest causes the following errors
inside the shim:

    d3v0 failed to reserve 512 extents of order 512 for offlining

And the ballooned-out pages stay inside shim and don't reach L0 Xen.

Fix this by passing the correct arguments to pv_shim_offline_memory()
during a XENMEM_decrease_reservation request.

Fixes: b2245acc60c3 ('xen/pvshim: memory hotplug')
Signed-off-by: Sergey Dyasli <sergey.dyasli@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Juergen Gross <jgross@suse.com>