Similar problem converting vmdk to qcow2
qemu-img: Could not open 'image.vmdk': Could not open backing file: Unknown protocol
I used export to ova/ovf in vmware which made a different vmdk output which qemu-img accepted and converted just fine.
Q: Is there a way to detect presence of snapshot from within VM guest OS*
A: not that I know. However, you can use the sg_vpd to check if UNMAP is supported. For example:
# sg_vpd /dev/sdb -p lbpv
Logical block provisioning VPD page (SBC):
Unmap command supported (LBPU): 1
Write same (16) with unmap bit supported (LBWS): 0
Write same (10) with unmap bit ...