Skip any files which are not mounted on the host

Skip any files which are not mounted on the host

Currently the LXC container tries to skip selinux/securityfs
mounts if the directory does not exist in the filesystem,
or if SELinux is disabled.

The former check is flawed because the /sys/fs/selinux
or /sys/kernel/securityfs directories may exist in sysfs
even if the mount type is disabled. Instead of just doing
an access() check, use an virFileIsMounted() to see if
the FS is actually present in the host OS. This also
avoids the need to check is_selinux_enabled().

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>