KVM: selftests: Fix number of pages for memory slot in memslot_modification_stress_test

It's required by vm_userspace_mem_region_add() that memory size
should be aligned to host page size. However, one guest page is
provided by memslot_modification_stress_test. It triggers failure
in the scenario of 64KB-page-size-host and 4KB-page-size-guest,
as the following messages indicate.

 # ./memslot_modification_stress_test
 Testing guest mode: PA-bits:40,  VA-bits:48,  4K pages
 guest physical test memory: [0xffbfff0000, 0xffffff0000)
 Finished creating vCPUs
 Started all vCPUs
 ==== Test Assertion Failure ====
   lib/kvm_util.c:824: vm_adjust_num_guest_pages(vm->mode, npages) == npages
   pid=5712 tid=5712 errno=0 - Success
      1	0x0000000000404eeb: vm_userspace_mem_region_add at kvm_util.c:822
      2	0x0000000000401a5b: add_remove_memslot at memslot_modification_stress_test.c:82
      3	 (inlined by) run_test at memslot_modification_stress_test.c:110
      4	0x0000000000402417: for_each_guest_mode at guest_modes.c:100
      5	0x00000000004016a7: main at memslot_modification_stress_test.c:187
      6	0x0000ffffb8cd4383: ?? ??:0
      7	0x0000000000401827: _start at :?
   Number of guest pages is not compatible with the host. Try npages=16

Fix the issue by providing 16 guest pages to the memory slot for this
particular combination of 64KB-page-size-host and 4KB-page-size-guest
on aarch64.

Fixes: ef4c9f4f ("KVM: selftests: Fix 32-bit truncation of vm_get_max_gfn()")
Signed-off-by: Gavin Shan <gshan@redhat.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20221013063020.201856-1-gshan@redhat.com