Check for bridge before trying to boot with it

[alfreb]

alfred@alfred-ubuntu-home:~/vmrunner$ boot ~/IncludeOS/hello_includeos.elf.bin.grub.img
[ WARNING ] Running with sudo
Error: argument "bridge43" is wrong: Device does not exist

qemu-system-x86_64: -netdev tap,id=net0,vhost=on,script=/home/alfred/vmrunner/bin/qemu-ifup,downscript=/home/alfred/vmrunner/bin/qemu-ifdown: network script /home/alfred/vmrunner/bin/qemu-ifup failed with status 65280

[ PROGRAM_FAILURE ] process exited

We should either check for the bridge ahead of time or hint that it can be created with --create-bridge if this happens.