qemu-char: Fix missed data on unix socket

Commit 812c1057 introduced HUP detection on unix and tcp sockets prior
to a read in tcp_chr_read. This unfortunately broke CloudStack 4.2
which relied on the old behaviour where data on a socket was readable
even if a HUP was present.

A working solution is to properly check the return values from recv,
handling a closed socket once there is no more data to read.

Also enable polling for G_IO_NVAL to ensure the callback is called
for all possible events as these should now be possible to handle
with the improved error detection.

Signed-off-by: Nils Carlson <pyssling@ludd.ltu.se>
Message-Id: <1437338396-22336-1-git-send-email-pyssling@ludd.ltu.se>
[Do not handle EINTR; use socket_error(). - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>