migration: equation is more proper than and to check LOADVM_QUIT

LOADVM_QUIT allows a command to quit all layers of nested loadvm loops,
while current return value check is not that proper even it works now.

Current return value check "ret & LOADVM_QUIT" would return true if
bit[0] is 1. This would be true when ret is -1 which is used to indicate
an error of handling a command.

Since there is only one place return LOADVM_QUIT and no other
combination of return value, use "ret == LOADVM_QUIT" would be more
proper.
Signed-off-by: NWei Yang <richardw.yang@linux.intel.com>
Message-Id: <20190718064257.29218-1-richardw.yang@linux.intel.com>
Reviewed-by: NDr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: NDr. David Alan Gilbert <dgilbert@redhat.com>

migration/savevm.c

@@ -2437,7 +2437,7 @@ retry:
         case QEMU_VM_COMMAND:
             ret = loadvm_process_command(f);
             trace_qemu_loadvm_state_section_command(ret);
-            if ((ret < 0) || (ret & LOADVM_QUIT)) {
+            if ((ret < 0) || (ret == LOADVM_QUIT)) {
                 goto out;
             }
             break;