migration: Pretty failures for postcopy on unsupported memory types

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2057267

This patchset backports a few patches that makes postcopy fails prettier when guest memory contains unsupported types (outside anon, shmem, hugetlbfs). Relevant patches:

fa45f8dab9 util/mmap-alloc: qemu_fd_getfs() cb9d8b8ce1 vl.c: Create late backends before migration object ae30b9b289 migration/postcopy: Detect file system on dest host 74c38cf7fd migration: Allow postcopy_ram_supported_by_host() to report err

I pulled in also a bunch of other patches (the recent reworks on options.c, atomic counters, etc.) to avoid conflicts. It just feels like a good timing at early stage to pull these in early too.