char: replace avail_connections

char: replace avail_connections

No need to count the users of a CharDriverState, it can rely on the fact
of whether there is a CharBackend associated or if there is enough space
in the muxer.

Simplify and fold chr_mux_new_fe() in qemu_chr_fe_init() since there is
a single user now. Also switch from fprintf to raising error instead.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20161022100951.19562-5-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>