Allow domain disk images on root-squash NFS to coexist with security driver.

Allow domain disk images on root-squash NFS to coexist with security driver.

(suggested by Daniel Berrange, tested by Dan Kenigsberg)

virStorageFileGetMetadata will fail for disk images that are stored on
a root-squash NFS share that isn't world-readable.
SELinuxSetSecurityImageLabel is called during the startup of every
domain (as long as security_driver != "none"), and it will propogate
the error from virStorageFileGetMetadata, causing the domain startup
to fail. This is, however, a common scenario when qemu is run as a
non-root user and the disk image is stored on NFS.

Ignoring this failure (which doesn't matter in this case, since the
next thing done by SELinuxSetSecurityImageLabel - setting the file
context - will also fail (and that function already ignores failures
due to root-squash NFS) will allow us to continue bringing up the
domain. The result is that we don't need to disable the entire
security driver just because a domain's disk image is stored on
root-squashed NFS.