Add multi-arch support

Currently the domain template as defined in domain.xml.erb hard-codes the arch
attribute under OS tag to x86_64. This is really not required since the
'arch' attribute is automatically populated by libvirt

`<type arch='x86_64'>hvm</type>`

This prevents using this plugin to manage non-x86_64 architecture like
Power(ppc64) and Arm.

This patch removes the 'arch' attribute from the domain template