vm.py: Add bpftrace and bcc to default arch packages

[Byte-Lab]

cmd_install() specifies a list of default arch packages that are installed when installing arch onto a VM. These packages contain non-critical but useful packages such as gcc, git, perf, etc.

Let's add bpftrace and bcc as well, which can be useful for diagnosing bugs in kernels.

[Byte-Lab]

@osandov Just FYI these packages have a dependency on clang. Letting you know in case you don't want the default image to be bloated with that