devman: allow multiple event read calls up to EOF

Read calls may be repeated by VFS if the user destination memory is
not mapped in.  Devman currently assumes that all reads are
successful, and uses this to track whether EOF has been reached for
a particular event, discarding it as soon as this happens.  Upon
repetition, this may result in lost events for devmand.

With this patch, devman discards events only once devmand reads the
EOF marker, which itself can never generate a user page fault.  The
result is that read calls for data can be repeated safely, without
the risk of losing events in the process.

Change-Id: I9dfdf7f8c8992a20a10302d79c3506e61f8564b0