qemu_process: Read errors from child
qemu_process: Read errors from child

https://bugzilla.redhat.com/show_bug.cgi?id=1035955

There's a window when starting a qemu process between fork() and exec()
during which we are doing things that may fail but not tunnelling the
error to the daemon. This is basically all within qemuProcessHook().
So whenever we fail in something, e.g. placing a process onto numa node,
users are left with:

    error: Child quit during startup handshake: Input/output error

while the original error is thrown into the domain log:

    libvirt:  error : internal error: NUMA memory tuning in 'preferred'
    mode only supports single node

Hence, we should read the log file and search for the error message and
report it to users.

Signed-off-by: Michal Privoznik <83d82aaba2eed257f4814b0c239c260c4caaadf0@redhat.com>

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