Read some features after connecting.

Signed-off-by: Owen Smith <owen.smith@citrix.com>
Re-read backing store features if/when they change.
Also causes these features to be read later (after Connected), which
counteracts blkback's writing of these features just before going
Connected (due to being unable to determine the supported level until
backing storage is attached).
Affected values:
"feature-barrier", "feature-discard", "feature-flush-cache", "removable"

Signed-off-by: Owen Smith <owen.smith@citrix.com>
Rework enabling/disabling of "feature-persistent"

Signed-off-by: Owen Smith <owen.smith@citrix.com>