qom: add support for qom objects in modules.
build some devices (qxl, virtio-gpu, ccid, usb-redir) as modules.
build braille chardev as module.

v2: more verbose comment for "build: fix device module builds" patch.

note: qemu doesn't rebuild objects on cflags changes (specifically
      -fPIC being added when code is switched from builtin to module).
      Workaround for resulting build errors: "make clean", rebuild.

# gpg: Signature made Tue 07 Jul 2020 14:42:16 BST
# gpg:                using RSA key 4CB6D8EED3E87138
# gpg: Good signature from "Gerd Hoffmann (work) <kraxel@redhat.com>" [full]
# gpg:                 aka "Gerd Hoffmann <gerd@kraxel.org>" [full]
# gpg:                 aka "Gerd Hoffmann (private) <kraxel@gmail.com>" [full]
# Primary key fingerprint: A032 8CFF B93A 17A7 9901  FE7D 4CB6 D8EE D3E8 7138

* remotes/kraxel/tags/modules-20200707-pull-request:
  chardev: enable modules, use for braille
  vga: build virtio-gpu as module
  vga: build virtio-gpu only once
  vga: build qxl as module
  usb: build usb-redir as module
  ccid: build smartcard as module
  build: fix device module builds
  qdev: device module support
  object: qom module support
  module: qom module support

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>