Running page_fault_test on a Cortex A72 fails with

Test: ro_memslot_no_syndrome_guest_cas
Testing guest mode: PA-bits:40,  VA-bits:48,  4K pages
Testing memory backing src type: anonymous
==== Test Assertion Failure ====
  aarch64/page_fault_test.c:117: guest_check_lse()
  pid=1944087 tid=1944087 errno=4 - Interrupted system call
     1	0x00000000004028b3: vcpu_run_loop at page_fault_test.c:682
     2	0x0000000000402d93: run_test at page_fault_test.c:731
     3	0x0000000000403957: for_each_guest_mode at guest_modes.c:100
     4	0x00000000004019f3: for_each_test_and_guest_mode at page_fault_test.c:1108
     5	 (inlined by) main at page_fault_test.c:1134
     6	0x0000ffff868e503b: ?? ??:0
     7	0x0000ffff868e5113: ?? ??:0
     8	0x0000000000401aaf: _start at ??:?
  guest_check_lse()

because we don't have a guest_prepare stage to check the presence of
FEAT_LSE and skip the related guest_cas testing, and we end-up failing in
GUEST_ASSERT(guest_check_lse()).

Add the missing .guest_prepare() where it's indeed required.
Signed-off-by: Zenghui Yu <yuzenghui@huawei.com>
Acked-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20231007124043.626-1-yuzenghui@huawei.comSigned-off-by: Oliver Upton <oliver.upton@linux.dev>