QStorageInfo: allow getting the info of pseudo filesystems too

They won't be listed in QStorageInfo::mountedVolumes, but we can now
obtain their info if you know their mountpoint. For example, on Linux:

 $ ./qstorageinfo /dev /proc /sys
 Filesystem (Type)            Size  Available BSize  Label            Mounted on
 devtmpfs (devtmpfs) RW    4029772    4029764  4096                   /dev
 proc (proc)         RW          0          0  4096                   /proc
 sysfs (sysfs)       RW          0          0  4096                   /sys

Change-Id: I3e15a26e0e424169ac2bffff1417ba2429d6d9f4
Reviewed-by: Ivan Komissarov <ABBAPOH@gmail.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>