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

[Richard W.M. Jones]

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.