char: introduce CharBackend

This new structure is meant to keep the details associated with a char
driver usage. On initialization, it gets a tag from the mux backend.
It can change its handlers thanks to qemu_chr_fe_set_handlers().

This structure is introduced so that all frontend will be moved to hold
and use a CharBackend. This will allow to better track char usage and
allocation, and help prevent some memory leaks or corruption.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20161022095318.17775-10-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
