hw/block: Let the NVMe emulated device be target-agnostic

hw/block: Let the NVMe emulated device be target-agnostic

Now than the non-target specific memory_region_msync() function
is available, use it to make this device target-agnostic.

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 20200508062456.23344-4-philmd@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>