cpu_hotplug: fix range to start from current cpus

Also parse "info cpus" to get current number of cpus, instead of relying on some
config variable. This makes the test more robust.

Also maxcpus is now added unconditionally to the kvm command line (this is a
hack in virttest/kvm_vm.py, need to better understand understand hlp for a clean solution)

Also fix a minor issue in crash handler
various fixes for IP address assignment with bridged networking

- For bridged networking, tapfd "fd=" in qemu CLI does not seem to work. Autotest
probably passes the wrong file descriptor. Ignore this parameter and really
fixed.

- Suppress VMIPAddressMissingError and VMAddressVerificationError (affects
bridged networking for sure, not sure about user networking)