resctrl: Add functions to work with resctrl allocations

resctrl: Add functions to work with resctrl allocations

With this commit we finally have a way to read and manipulate basic resctrl
settings.  Locking is done only on exposed functions that read/write from/to
resctrlfs.  Not in functions that are exposed in virresctrlpriv.h as those are
only supposed to be used from tests.

More information about how resctrl works:

  https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/x86/intel_rdt_ui.txt

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>