util: don't check for parallel iteration in hash-related functions

util: don't check for parallel iteration in hash-related functions

This is the responsability of the caller to apply the correct lock
before using these functions. Moreover, the use of a simple boolean
was still racy: two threads may check the boolean and "lock" it
simultaneously.

Users of functions from src/util/virhash.c have to be checked for
correctness. Lookups and iteration should hold a RO
lock. Modifications should hold a RW lock.

Most important uses seem to be covered. Callers have now a greater
responsability, notably the ability to execute some operations while
iterating were reliably forbidden before are now accepted.

Signed-off-by: Vincent Bernat <vincent@bernat.im>