webadmin: Don't allow migration when using host cpu

1."Use Host CPU" label is change to Pass-through Host Cpu

Its the actual behavior we use below, see libvirt doc[1]

2. make sure the only migration mode allowed is Non migratable

[1] http://libvirt.org/formatdomain.html#elementsCPU

Change-Id: Ifb736a69ff2a0a4995e18a5842a352d61f93bf00
Bug-Url: https://bugzilla.redhat.com/1052168
Signed-off-by: Roy Golan <rgolan@redhat.com>