agent: avoid failing to get disk stat when deal with gvfs-fuse

gvfs-fuse filesystem prevents other user from stat it. Even root can not
stat other user's gvfs-fuse fs. Stat on it fails then the whole disk
stat function fails.

This patch skips gvfs-fuse related filesystem when collecting disk stat.
Moreover, this patch try..except in a finner per fs granularity, so that
failing on stat one fs does not fails the whole operation.

Change-Id: I69c674904c2ad9cb508c5294591a8ba0813e4238
Signed-off-by: Zhou Zheng Sheng <zhshzhou@linux.vnet.ibm.com>