ui: edit VM boot device bad behaviour

When changing VMs first boot order second boot order reset to "none".
It should be only reset to none if both are equal.

Bug-Url: https://bugzilla.redhat.com/1057149
Change-Id: Ie478d042a0cd6ec090aba84fe3f752c21bccb940
Signed-off-by: Shahar Havivi <shaharh@redhat.com>