core: Remove volume & format check on import VM command

There's no reason to check validaty of the import VM parameters when not
using collapse snapshots since the image format will be determined by the
command itself according to the destination format

Change-Id: I06dc245db032a427d3aa36f62eaae8edcf93f657
Bug-Url: https://bugzilla.redhat.com/1129684
Signed-off-by: Tal Nisan <tnisan@redhat.com>