Description of problem:
When removing host from cluster and adding host with different CPU architecture or oldest CPU family, VM CPU type is still equal to removed CPU type instead of being changed to the new host CPU type.
When this issue occurs, VM cannot be run as long as there is value in VM OS type.

Bug is relevant for host replacement to an older CPU family and for changing host CPU type from Intel to AMD and vice versa.

Version-Release number of selected component (if applicable):
rhevm-3.5.6.2-0.1.el6ev (vt18.2)
qemu-kvm-rhev-0.12.1.2-2.479.el6_7.2.x86_64
sanlock-2.8-2.el6_5.x86_64
vdsm-4.16.29-1.el6ev.x86_64
libvirt-client-0.10.2-54.el6_7.2.x86_64

How reproducible:
100%

Steps to Reproduce:
1. Add AMD host to cluster and create Windows 10 VM on it (select VM OS type "Windows 10").
2. Power off VM, remove AMD host and add Intel host to cluster.
3. Try to run VM.

Actual results:
VM failed to run with an error message:
The guest OS doesn't support the following CPUs: opteron_g4, opteron_g3...

(this error message is relevant for a different bug (https://bugzilla.redhat.com/show_bug.cgi?id=1277214), but it also shows that VM CPU type does not changed to the new host CPU type.

Observing engine DB shows that cPU family remains AMD:
-bash-4.1$ psql engine postgres  -c 'select vm_name,vds_group_cpu_name from vms'
    vm_name     | vds_group_cpu_name  
----------------+---------------------
 win7_from_temp | Intel Conroe Family
 win10          | AMD Opteron G4
(2 rows)


Expected results:
VM CPU type should be cleared when removing host from cluster.

Additional info:
VDSM and engine logs attached.
Bug set to medium, because the usecase for replacing CPU architecture from AMD to Intel or vice versa or reducing to an older CPU family is rare.