KVM: selftests: Fix build regression by using accessor function

Fix the stable backport of commit 05c2224d ("KVM: selftests: Fix
number of pages for memory slot in memslot_modification_stress_test"),
which caused memslot_modification_stress_test.c build failures due to
trying to access private members of struct kvm_vm.

v6.0 commit b530eba1

 ("KVM: selftests: Get rid of
kvm_util_internal.h") and some other commits got rid of the accessors
and made all of the KVM data structures public. Keep using the accessors
in older kernels.

There is no corresponding upstream commit for this change.

Signed-off-by: Tyler Hicks (Microsoft) <code@tyhicks.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>