start / shutdown should not assume there are usb device mappings

[mstovenour]

Need to thoroughly test a VM configuration where there are no usbHostDev entries

Also test case where usbHostDev entries exist but none are currently present:

virt-compose start example_vm1
...
Warning:  Could not locate device -> name: usb-FTDI_TTL232R-3V3_FTF50XXM
Warning:  Continuing without device
Warning:  Could not locate device -> name: usb-FTDI_TTL232R_FTDPY3VD
Warning:  Continuing without device
Info: Creating udev rules file: /etc/udev/rules.d/90-virt-compose-example_vm1.rules

virt-compose shutdown example_vm1
...
Info: Removing udev rules file: /etc/udev/rules.d/90-virt-compose-example_vm1.rules
rm: cannot remove '/etc/udev/rules.d/90-virt-compose-example_vm1.rules': No such file or directory
Error: Failed to remove /etc/udev/rules.d/90-virt-compose-example_vm1.rules