qemu: caps: Add capability for 'write-cache' parameter of disk frontends

qemu: caps: Add capability for 'write-cache' parameter of disk frontends

QEMU translates the cache mode of a disk internally into 3 flags.
'write-cache' is a flag of the frontend while others are flag of the
backing storage. Add capability which will allow expressing it via the
frontend attribute.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>