virCgroupV2DevicesAvailable: Print stringified errno in the debug log
virCgroupV2DevicesAvailable: Print stringified errno in the debug log

In the virCgroupV2DevicesAvailable() function we try to determine
whether CGroups version 2 are available. We do this by opening
what we believe is the CGroup mount point and issuing a BPF call.
When the call fails, a debug message is printed. However, the BPF
call sets errno too. Include it in the debug message to help us
with debugging.

Signed-off-by: Michal Privoznik <83d82aaba2eed257f4814b0c239c260c4caaadf0@redhat.com>
Reviewed-by: Daniel Henrique Barboza <627b089ad62deae9aaf06ccfa2c9056df28927e0@gmail.com>

olafhering/libvirt,olafhering/libvirt,libvirt/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,libvirt/libvirt,jfehlig/libvirt,zippy2/libvirt,nertpinx/libvirt,jfehlig/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,nertpinx/libvirt,crobinso/libvirt,jardasgit/libvirt,zippy2/libvirt,jfehlig/libvirt,crobinso/libvirt,libvirt/libvirt,crobinso/libvirt,nertpinx/libvirt,jardasgit/libvirt,nertpinx/libvirt,fabianfreyer/libvirt,fabianfreyer/libvirt,fabianfreyer/libvirt,libvirt/libvirt,crobinso/libvirt,zippy2/libvirt,jfehlig/libvirt,olafhering/libvirt,olafhering/libvirt,zippy2/libvirt,nertpinx/libvirt,jardasgit/libvirt