virsh: cmdAttachDisk: Don't assume type='block' if file can't be examined

virsh: cmdAttachDisk: Don't assume type='block' if file can't be examined

'virsh attach-disk' uses stat() to determine if the 'source' is a
regular file. If stat fails though it assumes that the file is block.

Since it's way more common to have regular files and the detection does
not work at all when accessing a remote host, modify the default to
assume type='file' by default.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>