KVM: selftests: Make the per vcpu memory size global

Rename vcpu_memory_bytes to something with "percpu" in it
in order to be less ambiguous. Also make it global to
simplify things.

Signed-off-by: Andrew Jones <drjones@redhat.com>
Message-Id: <20201104212357.171559-7-drjones@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>