memory: Display MemoryRegion name in read/write ops trace events

MemoryRegion names is cached on first call to memory_region_name(),
so displaying the name is trace events is cheap. Add it for read /
write ops.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20210307074833.143106-1-f4bug@amsat.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>