libvirt.rb: fix smbios handling

In case smbios isn't set the current code sets parameter to nil. This
raise an error when calling parameters.each. Add a check around the
code block to prevent the not needed assignment.

Signed-off-by: Danny Al-Gaaf <d.al-gaaf@telekom.de>
Merge pull request #3 from TelekomCloud/fix/not-set-smbios-parameter

libvirt.rb: fix smbios handling