chardev: add socket chardev support to chardev-add (qmp)

qemu_chr_open_socket is split into two functions.  All initialization
after creating the socket file handler is split away into the new
qemu_chr_open_socket_fd function.

chr->filename doesn't get filled from QemuOpts any more.  Qemu gathers
the information using getsockname and getnameinfo instead.  This way it
will also work correctly for file handles passed via file descriptor
passing.

Finally qmp_chardev_open_socket() is the actual qmp hotplug
implementation which basically just calls socket_listen or
socket_connect and the new qemu_chr_open_socket_fd function.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>