qemu: Only raise memlock limit if necessary

qemu: Only raise memlock limit if necessary

Attempting to set the memlock limit might fail if we're running
in a containerized environment where CAP_SYS_RESOURCE is not
available, and if the limit is already high enough there's no
point in trying to raise it anyway.

https://bugzilla.redhat.com/show_bug.cgi?id=1916346

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>