block/curl: Replace usleep by g_usleep

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: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>