Bug 26996 — wrongly detects partition on unpartitioned VFAT device

Bug 26996 — wrongly detects partition on unpartitioned VFAT device

A standard VFAT header matches the signature of an MBR, so we detected
VFAT file systems on raw unpartitioned devices as both VFAT and MBR partition
table. This confuses gnome-disk-utility/gvfs, and causes them to be ignored in
GNOME.

The standard approach in the kernel/blkid is to only detect an MBR if it does
not have a valid VFAT signature. Since blkid already does that for us, just
skip the udisks-part-id invocation completely if we already know that we have a
valid ID_FS_TYPE.