cpus: Fix configure_icount() error API violation

cpus: Fix configure_icount() error API violation

The Error ** argument must be NULL, &error_abort, &error_fatal, or a
pointer to a variable containing NULL.  Passing an argument of the
latter kind twice without clearing it in between is wrong: if the
first call sets an error, it no longer points to NULL for the second
call.

configure_icount() is wrong that way.  Harmless, because its @errp is
always &error_abort or &error_fatal.

Just as wrong (and just as harmless): when it fails, it can still
update global state.

Fix all that.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20200422130719.28225-4-armbru@redhat.com>