KVM: selftests: Add a macro to iterate over a sparsebit range

Add sparsebit_for_each_set_range() to allow iterator over a range of set
bits in a range.  This will be used by x86 SEV guests to process protected
physical pages (each such page needs to be encrypted _after_ being "added"
to the VM).

Tested-by: Carlos Bilbao <carlos.bilbao@amd.com>
Signed-off-by: Ackerley Tng <ackerleytng@google.com>
[sean: split to separate patch]
Link: https://lore.kernel.org/r/20240223004258.3104051-4-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>