Add chardev API qemu_chr_fe_read_all

Add chardev API qemu_chr_fe_read_all

This function will attempt to read data from the chardev trying
to fill the buffer up to the given length.
Add tcp_chr_disconnect to reuse disconnection code where needed.

Signed-off-by: Antonios Motakis <a.motakis@virtualopensystems.com>
Signed-off-by: Nikolay Nikolaev <n.nikolaev@virtualopensystems.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>