qemu-char: introduce qemu_chr_alloc

The next patch will modify this function to initialize state that is
common to all backends.

Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
