compilation failure on macOS caused by src/usb-device-cd.c

As referred to in a previous issue, attempting to build on macOS fails with "fatal error: 'linux/fs.h' file not found" because the includes in src/usb-device-cd.c only account for either Windows or Linux. This came up while updating the Homebrew formula for spice-gtk. For now, passing -Dusbredir=disabled routes around the problem.

Another issue that came up was how meson will not use the correct Python environment unless subprojects/spice-common/meson.build has its line py_module.find_installation() changed to py_module.find_installation('python3'). The formula creates a fresh virtual environment with the two Python packages installed and adds its binaries to the PATH, but without the above change it attempts to use the stock one in /usr/local/opt.