iotests: Drop use of bash keyword 'function'

iotests: Drop use of bash keyword 'function'

Bash allows functions to be declared with or without the leading
keyword 'function'; but including the keyword does not comply with
POSIX syntax, and is confusing to ksh users where the use of the
keyword changes the scoping rules for functions.  Stick to the
POSIX form through iotests.

Done mechanically with:
  sed -i 's/^function //' $(git ls-files tests/qemu-iotests)

Signed-off-by: Eric Blake <[email protected]>
Message-Id: <20181116215002.2124581[email protected]>
Reviewed-by: Daniel P. Berrangé <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>