qtest: Use cpu address space instead of system memory

Some devices (like nvic in armv7m) are not accessable through
address_space_memory, therefore can not be tested with qtest.

Signed-off-by: Julia Suvorova <jusual@mail.ru>
Message-Id: <20180702065237.27899-1-jusual@mail.ru>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>