Simplify interface: Device.t

Device.t is now exposed as a purely abstract type, and is not wrapped in
Ctypes.structure in the module interface.

Device.internal_t replaces the previous Device.t

Signed-off-by: John Else <john.else@citrix.com>
Simplify interface: Memory.t

Memory.t is now a native OCaml record type.

Memory.internal_t is the type which is passed to C, and is no longer
exposed in the module interface.

Signed-off-by: John Else <john.else@citrix.com>
Simplify interface: Utilization.t

Utilization.t is now a native OCaml record type.

Signed-off-by: John Else <john.else@citrix.com>
Fix Device.get_vbios_version

Signed-off-by: John Else <john.else@citrix.com>