usb: fix some memory allocation issues (CVE-2021-3527).
usb: add stubs, allow building without usb subsystem.

# gpg: Signature made Wed 05 May 2021 14:07:02 BST
# gpg:                using RSA key A0328CFFB93A17A79901FE7D4CB6D8EED3E87138
# 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/usb-20210505-pull-request:
  usb: limit combined packets to 1 MiB (CVE-2021-3527)
  usb/mtp: avoid dynamic stack allocation
  usb/redir: avoid dynamic stack allocation (CVE-2021-3527)
  usb/hid: avoid dynamic stack allocation
  hw/usb: Do not build USB subsystem if not required
  hw/usb/host-stub: Remove unused header

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