hw: Do not include "sysemu/block-backend.h" if it is not necessary

Remove those unneeded includes to speed up the compilation
process a little bit. (Continue 7eceff5b cleanup)

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20180528232719.4721-13-f4bug@amsat.org>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>