vvfat: Fix segfault on write to read-only disk

vvfat tries to set the readonly flag in its open function, but nowadays
this is overwritted with the readonly=... command line option. Check in
bdrv_write if the vvfat was opened read-only and return an error in this
case.

Without this check, vvfat tries to access the qcow bs, which is NULL
without enabled write support.

Signed-off-by: Kevin Wolf <mail@kevin-wolf.de>
