Revert "tools/libxl: Adjust datacopiers POLLHUP handling when the fd is also readable"

Revert "tools/libxl: Adjust datacopiers POLLHUP handling when the fd is also readable"

The bootloader code is relying on detecting POLLHUP, and 7e9ec50b
breaks that.  7e9ec50b, when handling a pty master, violates the
specification of the datacopier interface (as defined).

When the bootloader exits, several things change, all at once:
 (a) The master pty fd (held by libxl) starts to signal POLLHUP
    and maybe also POLLIN.
 (b) The child exits (so that the SIGCHLD self-pipe signals POLLIN,
    which will be handled by the libxl child process code.
 (c) reads on the master pty fd start to return EOF

From the point of view of the datacopier these might happen in any
order.  I think there is a latent bug with (c), which I will discuss
later in this email.

In a recent bug report from a FreeBSD installation, the datacopier
gets told about (a) before (b).  But 7e9ec50b filters the POLLHUP out,
so that the dc signals eof rather than hup.  As a result in
bootloader_copyfail we take the error path.

This reverts commit 7e9ec50b0535bf2630da9d279a060775817d136d.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
CC: Andrew Cooper <andrew.cooper3@citrix.com>
CC: Roger Pau Monné <roger.pau@citrix.com>
CC: Ian Campbell <ian.campbell@citrix.com>
CC: Ross Lagerwall <ross.lagerwall@citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>