qga: Fix qmp_guest_suspend_{disk, ram}() error handling

qga: Fix qmp_guest_suspend_{disk, ram}() error handling

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

qmp_guest_suspend_disk() and qmp_guest_suspend_ram() pass @local_err
first to check_suspend_mode(), then to acquire_privilege(), then to
execute_async().  Continuing after errors here can only end in tears.
For instance, we risk tripping error_setv()'s assertion.

Fixes: aa59637ea1c6a4c83430933f9c44c43e6c3f1b69
Fixes: f54603b6aa765514b2519e74114a2f417759d727
Cc: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20200422130719.28225-15-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>