chardev: Properly initialize ChardevCommon components

Commit d0d7708b forgot to parse logging for spice chardevs and
virtual consoles. This requires making qemu_chr_parse_common()
non-static. While at it, use a temporary variable to make the
code shorter, as well as reduce the churn when a later patch
alters the layout of simple unions.

Signed-off-by: Eric Blake <eblake@redhat.com>
CC: Daniel P. Berrange <berrange@redhat.com>
Message-Id: <1455927587-28033-2-git-send-email-eblake@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>