Revert "qemu-char: Keep pty slave file descriptor open until the master is closed"

Revert "qemu-char: Keep pty slave file descriptor open until the master is closed"

This reverts commit 34689e206abddac87a5217d458534e24f2a05562.

Marc-André Lureau provided the following commentary: "It looks like if
a the slave is opened, then Linux will buffer the master writes, up to
a few kb and then throttle, so it's not entirely blocked but eventually
the guest VM dies.  However, not having any slave open it will simply let
the write go and discard the data.  At least, virt-install configures
a pty for the serial but viewers like virt-manager do not necessarily
open it.  And, if there are no viewers, it will just hang.  If qemu
starts reading all the data from the slave, I don't think interactions
with other slaves will work. I don't see much options but to close the
slave, thus reverting this patch."

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>