selftests: kvm: fix overlapping addresses in memslot_perf_test

vm_create allocates memory and maps it close to GPA.  This memory
is separate from what is allocated in subsequent calls to
vm_userspace_mem_region_add, so it is incorrect to pass the
test memory size to vm_create_default.  Just pass a small
fixed amount of memory which can be used later for page table,
otherwise GPAs are already allocated at MEM_GPA and the
test aborts.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>