[qemu]: Fix payload size format in qemu_pipe_frame_send()

The following change introduced regression for booting AOSP
on emulator for all architectures:

7c7990e6 Fix warnings in system/ headers

qemu_pipe expects payload size in the message header to be in hex format,
as stated in the comment for qemu_pipe_frame_send().

Change-Id: I32cfa9c9b2af56f93bbda0ea5597451c389ae45e
Signed-off-by: Miodrag Dinic <miodrag.dinic@imgtec.com>