stat: Add descriptions for new virtio_balloon stat types

Unknown types would print the value with no descriptive text at all.
Add descriptions for all known stat types, and a default description
when the type is unknown.

Signed-off-by: Keir Fraser <keirf@google.com>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20220520143706.550169-3-keirf@google.com


Signed-off-by: Will Deacon <will@kernel.org>