gluster: default scheme to gluster:// and host to localhost.

Currently, "gluster:///volname/img" and (using file. options)
"file.driver=gluster,file.filename=foo" will segfault.  Also,
"//host/volname/img" will be rejected, but it is a valid URL
that should be accepted just fine with "file.driver=gluster".
Accept all of these, by inferring missing transport and host
as TCP and localhost respectively.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
