engine:  VM Linux Boot options aren't parsing properly

Steps to Reproduce:
1. have VM & init & kernel file on ISO domain
2. use Boot params =>  kernel/initrd params
3. put to initrd: "iso://rhel6.initrd " (without quotes, with space at the end)

After starting the VM it runs for a moment and then it crashes with VDSM error:
libvirtError('virDomainCreateXML() failed', conn=self).
Fixed by adding validation of the user input (on frontend) and setting of
VmBase parameters (on backend).

Change-Id: I9ccb4fc0b807c77bc17cfe02903341cdf4e51d54
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=872178