KVM: selftests: Drop @mode from common vm_create() helper

Drop @mode from vm_create() and have it use VM_MODE_DEFAULT.  Add and use
an inner helper, __vm_create(), to service the handful of tests that want
something other than VM_MODE_DEFAULT.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>