hw: Add test device for unittests execution

hw: Add test device for unittests execution

Add a test device which supports the kvmctl ioports,
so one can run the KVM unittest suite.

Intended Usage:

qemu-system-x86_64 -nographic \
    -device pc-testdev \
    -device isa-debug-exit,iobase=0xf4,iosize=0x04 \
    -kernel /path/to/kvm/unittests/msr.flat

Where msr.flat is one of the KVM unittests, present on a
separate repo,

git://git.kernel.org/pub/scm/virt/kvm/kvm-unit-tests.git

[ kraxel: more memory api + qom fixes ]

CC: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>