luaengine: add memory-reading capabilities

This commit maps some more classes (device_t and address_space)
to retrieve devices and address spaces out of current running
machine.
Proxy methods are provided to enumerate/access devices and
address spaces, and to read memory content.

The idea is to provide user scripts some means to introspect on
current machine status.

Signed-off-by: Luca Bruno <lucab@debian.org>