ignore SELinuxSetFilecon error in SELinuxSetSecurityFileLabel if on nfs

ignore SELinuxSetFilecon error in SELinuxSetSecurityFileLabel if on nfs

If virDomainAttachDevice() was called with an image that was located
on a root-squashed NFS server, and in a directory that was unreadable
by root on the machine running libvirtd, the attach would fail due to
an attempt to change the selinux label of the image with EACCES (which
isn't covered as an ignore case in SELinuxSetFilecon())

NFS doesn't support SELinux labelling anyway, so we mimic the failure
handling of commit 93a18bbafaf11729d3ca1241e11bee133d77fa77, which
just ignores the errors if the target is on an NFS filesystem (in
SELinuxSetSecurityAllLabel() only, though.)

This can be seen as a follow-on to commit
347d266c51705f4987fa5ce2a0ecb314ed8745ce, which ignores file open
failures of files on NFS that occur directly in
virDomainDiskDefForeachPath() (also necessary), but does not ignore
failures in functions that are called from there (eg
SELinuxSetSecurityFileLabel()).