util: tell users that memory locking ulimit is too low for BPF

util: tell users that memory locking ulimit is too low for BPF

If running libvirtd via systemd, it gets a 64 MB memlock limit, but if
running from the shell it will only get 64 KB on a Fedora 33 system.
The latter low limit causes any attempt to use BPF to fail and it is
not obvious why.

This improves the error message thus:

  # virsh -c lxc:/// start sh
error: Failed to start domain 'sh'
error: internal error: guest failed to start: Failure in libvirt_lxc startup: failed to initialize device BPF map; locked memory limit for libvirtd probably needs to be raised: Operation not permitted

Reviewed-by: Pavel Hrdina <phrdina@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>