Allow qb_ipcc_event_recv to receive pointer to shm file

RFE. Current qb_ipcc_event_recv requires caller to pass already allocated buffer. In corosync, we are passing 1M allocated on stack. This makes problems in environments with limited stack size. One solution is to allocate on heap. Even better solution seems to be used in corosync 1.x. Instead of memcpy of buffer from /dev/shm/file to provided preallocated size, it would be nice to have option (probably only for shm based IPC) to get direct pointer to shm file. This would make ipc to not only require additional space but also a little bit faster.