KVM: selftests: Fix 32-bit truncation of vm_get_max_gfn()

vm_get_max_gfn() casts vm->max_gfn from a uint64_t to an unsigned int,
which causes the upper 32-bits of the max_gfn to get truncated.

Nobody noticed until now likely because vm_get_max_gfn() is only used
as a mechanism to create a memslot in an unused region of the guest
physical address space (the top), and the top of the 32-bit physical
address space was always good enough.

This fix reveals a bug in memslot_modification_stress_test which was
trying to create a dummy memslot past the end of guest physical memory.
Fix that by moving the dummy memslot lower.

Fixes: 52200d0d ("KVM: selftests: Remove duplicate guest mode handling")
Reviewed-by: Venkatesh Srinivas <venkateshs@chromium.org>
Signed-off-by: David Matlack <dmatlack@google.com>
Message-Id: <20210521173828.1180619-1-dmatlack@google.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>