GetBlockInfo: Use the correct path to qemuOpenFile

GetBlockInfo: Use the correct path to qemuOpenFile

Commit id '3ea661de' refactored the code to use the 'disk->src->path'
instead of getting the path from virDomainDiskGetSource().  The one
call to qemuOpenFile() didn't use the disk source path, rather it used
the path as passed from the caller (in this case 'vda') - this caused
a failure with the virt-test/tp-libvirt as follows:

$ virsh domblkinfo virt-tests-vm1 vda
error: cannot stat file '/home/virt-test/shared/data/images/jeos-20-64.qcow2': Bad file descriptor

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