util: bitmap: Don't alloc overly large binary bitmaps

util: bitmap: Don't alloc overly large binary bitmaps

Optimize the virBitmap to array-of-char bitmap conversion by skipping
trailing zero bytes.

This also fixes a regression when requesting iothread information from a
live VM since after commit 825df8c3158cfaf5f398418471f10f4ff3c3515a the
bitmap returned from virProcessGetAffinity is too big to be formatted
properly via RPC. A user would get the following error:

error: Unable to get domain IOThreads information
error: Unable to encode message payload

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1238589