Improve checks to make sure loop and dm-mod module are present

This fix was triggered because running --vmfile to create a virtual
machine image file caused the following error:

| device-mapper: reload ioctl on loop1p1p1 failed: Device or resource busy
| create/reload failed on loop1p1p1

This was caused because kpartx was run twice. The first run was used to test if
the Device-mapper was ready and the second run was actually add the partition
mappings. This used to fail under some circumstances.