Dev Build

block: Add UnsupportedFlags error variant for flag validation

Introduce ExecuteError::UnsupportedFlags to carry both the
request type and the rejected flags value, replacing the
generic ExecuteError::Unsupported at discard and write zeroes
flag validation sites. This provides structured context for
debugging without changing the returned VIRTIO_BLK_S_UNSUPP
status.

Signed-off-by: Anatol Belski <anbelski@linux.microsoft.com>