test: add some chardev mux event tests

Check the expected behaviour of qemu_chr_be_event() on a mux chardev.

For some reason, sending the event on the base chardev broadcast to
all frontends, while sending it on the mux chardev itself should
trigger the event on the currently focused chardev frontend.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20171103152824.21948-3-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
