qemu-img: Move quiet into ImgConvertState

qemu-img: Move quiet into ImgConvertState

Move img_convert()'s quiet flag into the ImgConvertState so it is
accessible by nested functions.  -q dictates that it suppresses anything
but errors, so if those functions want to emit warnings, they need to
query this flag first.  (There currently are no such warnings, but there
will be as of the next patch.)

Signed-off-by: Max Reitz <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Reviewed-by: Vladimir Sementsov-Ogievskiy <[email protected]>
Message-id: 20190507203508[email protected]
Signed-off-by: Max Reitz <[email protected]>