qemu-char: simplify pty polling

There is no need to use a timer and pty_chr_read to detect a connected
pty.  It is simpler to just call g_poll periodically and check for POLLHUP.
It is done once per second, and only if the pty is disconnected, so it
is cheap enough.

Tested with "-monitor pty" and "-serial mon:pty", both of which work
correctly and do not freeze QEMU.  (How to test ptys?  "socat -,raw,echo=0
/dev/pts/4,raw").

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 1366385529-10329-3-git-send-email-pbonzini@redhat.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
