xen, pvh: fix unbootable VMs by inlining memset() in xen_prepare_pvh()

If this memset() is not inlined than PVH early boot code can call
into KASAN-instrumented memset() which results in unbootable VMs.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Acked-by: Juergen Gross <jgross@suse.com>
Message-ID: <20240802154253.482658-3-adobriyan@gmail.com>
Signed-off-by: Juergen Gross <jgross@suse.com>