core: use instance type values on update vm

When updating vm which is using instance-type,
the instance-type values cannot be override by the user.

this patch use the same code we have for add-vm that use instance-type,
to override the correct values.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1136372
Change-Id: Ib3ed838af7052f744dccd015f90032fdfc18cc55
Signed-off-by: Omer Frenkel <ofrenkel@redhat.com>