Add support for detecting the partition table type when scanning

Add support for detecting the partition table type when scanning
iSCSI volumes.  This is implemented in the
virStorageBackendUpdateVolInfoFD function, so all future callers will
automatically benefit.  This is a somewhat large patch because the
conversion of the virStorageBackendPartTableTypeToString necessitated
a change to the formatToString and formatFromString function pointers,
which caused fallout in other places in the storage stuff.  The good
news is that most of these callers are now converted over to the
VIR_ENUM_IMPL, which means a lot of redundant code is now gone.

Signed-off-by: Chris Lalancette <clalance@redhat.com>