xen, pvh: fix unbootable VMs (PVH + KASAN - AMD_MEM_ENCRYPT)

Uninstrument arch/x86/platform/pvh/enlighten.c: KASAN has not been setup
_this_ early in the boot process.

Steps to reproduce:

make allnoconfig
make sure CONFIG_AMD_MEM_ENCRYPT is disabled
AMD_MEM_ENCRYPT independently uninstruments lib/string.o
so PVH boot code calls into uninstrumented memset() and
memcmp() which can make the bug disappear depending on
the compiler.
enable CONFIG_PVH
enable CONFIG_KASAN
enable serial console
this is fun exercise if you never done it from nothing :^)

make

qemu-system-x86_64 \
-enable-kvm \
-cpu host \
-smp cpus=1 \
-m 4096 \
-serial stdio \
-kernel vmlinux \
-append 'console=ttyS0 ignore_loglevel'

Messages on serial console will easily tell OK kernel from unbootable
kernel. In bad case qemu hangs in an infinite loop stroboscoping
"SeaBIOS" message.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Acked-by: Juergen Gross <jgross@suse.com>
Message-ID: <20240802154253.482658-1-adobriyan@gmail.com>
Signed-off-by: Juergen Gross <jgross@suse.com>