core: Determine block storage type by user.

Importing a Storage Domain should be determined by the user instead it
will be determined from the meta data.

Change-Id: I6d5076d78eea0960da41a058b4de17bbb01b05f1
Bug-Url: https://bugzilla.redhat.com/1157240
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>