block/curl: Replace usleep by g_usleep

The function usleep is not available for all supported platforms:
at least some versions of MinGW don't support it.

usleep was also declared obsolete by POSIX.1-2001.

The function g_usleep is part of glib2.0, so it is available for
all supported platforms.

Using nanosleep would also be possible but needs more code.
Signed-off-by: NStefan Weil <sw@weilnetz.de>
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>

block/curl.c

@@ -282,7 +282,7 @@ static CURLState *curl_init_state(BDRVCURLState *s)
             break;
         }
         if (!state) {
-            usleep(100);
+            g_usleep(100);
             curl_multi_do(s);
         }
     } while(!state);