Currently, keep current behaviour by always using get_system_memory().

Also use QOM casts when possible.
Signed-off-by: NHervé Poussineau <hpoussin@reactos.org>
Reviewed-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NLeon Alrae <leon.alrae@imgtec.com>