frontend: VM Linux Boot options aren't parsing properly

- added checking Linux Boot options in the Run Once dialog.
- slightly refactored validity checking to improve code readibility
- also fixed the error when the invalidity messages in "kernel params" and
  "initrd path" fields weren't displayed (caused by setting invalidity
  flag before setting invalidity message)

Change-Id: I23bd3ee3677c9bdbfacf1cf341dc1d97fd57c65e
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=872178