exec: Rename qemu_ram_writeback() as qemu_ram_msync()

exec: Rename qemu_ram_writeback() as qemu_ram_msync()

Rename qemu_ram_writeback() as qemu_ram_msync() to better
match what it does.

Suggested-by: Stefan Hajnoczi <stefanha@redhat.com>
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: 20200508062456.23344-5-philmd@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>