migration: Assert that migrate_multifd_compression() returns an in-range value

migration: Assert that migrate_multifd_compression() returns an in-range value

Coverity complains that when we use the return value from
migrate_multifd_compression() as an array index:
  multifd_recv_state->ops = multifd_ops[migrate_multifd_compression()];

that this might overrun the array (which is declared to have size
MULTIFD_COMPRESSION__MAX).  This is because the function return type
is MultiFDCompression, which is an autogenerated enum.  The code
generator includes the "one greater than the maximum possible value"
MULTIFD_COMPRESSION__MAX in the enum, even though this is not
actually a valid value for the enum, and this makes Coverity think
that migrate_multifd_compression() could return that __MAX value and
index off the end of the array.

Suppress the Coverity error by asserting that the value we're going
to return is within range.

Resolves: Coverity CID 1487239, 1487254
Signed-off-by: Peter Maydell <[email protected]>
Message-Id: <20220721115207[email protected]>
Reviewed-by: Dr. David Alan Gilbert <[email protected]>
Reviewed-by: Juan Quintela <[email protected]>
Signed-off-by: Dr. David Alan Gilbert <[email protected]>