plugins: Reorg arguments to qemu_plugin_vcpu_mem_cb

plugins: Reorg arguments to qemu_plugin_vcpu_mem_cb

Use the MemOpIdx directly, rather than the rearrangement
of the same bits currently done by the trace infrastructure.
Pass in enum qemu_plugin_mem_rw so that we are able to treat
read-modify-write operations as a single operation.

Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>