core: introduce AddVmParameters

As a step in the way for having one parameters class for AddVmCommand +
AddVmFromTemplate + AddVmFromScratch, AddVmFromScratchParameters is
renamed to AddVmParameters.

Change-Id: Ia5b2f28897c6017a38e30db88918af6225e1cb3d
Signed-off-by: Arik Hadas <ahadas@redhat.com>