qemu builder - support deleting arguments

This addresses an issue when using the qemu builder with older versions of Qemu by allowing specific arguments to be removed before Qemu is executed.  Specifically this was needed to remove the "-display' option, which doesn't seem to exist in the version of Qemu found on RHEL6.x variants.  Additionally we allow virtio-net-pci as a valid network interface type to support older versions of Qemu.

I'm happy to address this using a different method/approach.  This touches none of the existing code so it seemed clean.

Example usage:
      "delete_qemuargs":
      [
        "-display"
      ]