[Libguestfs] [RFC] Inconsistent output of guestfs_list_filesystems

Before I rush to change something I request your comments on the subject. Let me know what do you think and if it does make sense. The issue: guesfs_list_filesystems is inconsistent in its output. For, example, it filters out partitioned physical devices but doesn't do the same for MD devices. More over, according to its name and API documentation guestfs_list_filesystem should return something which potentially can be mounted (SWAP is an exclusion and it is stated in API doc). It is the reason partitioned physical devices are filtered out: they are not mountable even in theory (they don't contain filesystem if they contain partition(s)). Another example of non-mountable block device is extended MSDOS partition. Also nothing stops you from creating partition table and partitions on logical volume (yes, it is probably uncommon but nevertheless). In this case such partitions are ignored completely. Yet another example is Windows dynamic disks aka LDM: guestfs_list_filesystems returns both ldm_vol_ and ldm_part_ but strongly speaking only volumes should be considered as mountables. And last example is again with Windows dynamic disks and their physical counterparts: guestfs_list_filesystems filters out disks with MBR type byte 0x42 but what about GPT? Patch #1 will add one more test to demonstrate one of the issues described above (partitioned MD device). Patch #2 will define function which tells if device is partitioned (yes, it doesn't work with dm-devices). Patch #3 fixes the issue with partitioned MD devices. At the moment don't pay too much attention at the code itself - it is just a quick way to demonstrate the issue.