core: cleanup in RunVmCommand

1. remove '_' prefix from class fields

2. define a static field that stores "iso://" string

3. use #cdPathWindowsToLinux method instead of calling
ImagesHandler.cdPathWindowsToLinux explicitly in some places.

4. reference RunVmCommand#ISO_PREFIX from the test class, instead of
defining the iso prefix string again in the test class

5. use IsoDomainListSyncronizer#findActiveISODomain to find active iso
domain instead of having duplicated code

6. inline fields in canDoAction method

7. separate network interfaces validations in canDoAction method from
the validator checks to make it more clear

Change-Id: Ia37a78eacd6c415a9bf10920362140736c7d70fc
Signed-off-by: Arik Hadas <ahadas@redhat.com>