[Libguestfs] lib: Convert all drive socket parameters to an absolute path

One interesting omission is that we don't allow sockets in the abstract namespace. The API won't let you pass these sockets because they contain a '\0' character in the middle of the string. Therefore this patch doesn't need to deal with those. However we should in future allow that, probably using the '@' character to stand in for the NUL byte, as is used in a few other places (eg. this is used by the ‘netstat’ program). Rich.