build: don't build hardware objects with linux-user

Some objects are only needed for system emulation and tools.
We can ignore them for the user mode case

Update tests to run accordingly: conditionally build some tests
on CONFIG_BLOCK.

Some tests use components that are only built when softmmu or
block tools are enabled, not for linux-user. So, if these components
are not available, disable the tests.

Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20190401141222.30034-6-lvivier@redhat.com>
Signed-off-by: Laurent Vivier <lvivier@redhat.com>