qemu-char: add create to register_char_driver

qemu-char: add create to register_char_driver

Having creation as a member of the CharDriver struct removes the need
to export functions for qemu-char.c's usage.  After the conversion,
chardev backends implemented outside qemu-char.c will not need a stub
creation function anymore.

Ultimately all drivers will be converted.  For now, support the case
where cd->create == NULL.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>