hw/sd/milkymist-memcard: Reset SD card on controller reset
    
    Since milkymist-memcard is still using the legacy SD card API,
    the SD card created by sd_init() is not plugged into any bus.
    This means that the controller has to reset it manually.
    
    Failing to do this mostly didn't affect the guest since the
    guest typically does a programmed SD card reset as part of
    its SD controller driver initialization, but meant that
    migration failed because it's only in sd_reset() that we
    set up the wpgrps_size field.
    
    Cc: qemu-stable@nongnu.org
    Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
    Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
    Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
    Message-id: 1515506513-31961-3-git-send-email-peter.maydell@linaro.org
    (cherry picked from commit 16bf0e0e7aaa8efc0b8ee7e2aecb2fa235f82d38)
    Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>