blockdev: preserve readonly and snapshot states across media changes

blockdev: preserve readonly and snapshot states across media changes

If readonly=on is given at device creation time, the ->readonly flag
needs to be set in the block driver state for this device so that
readonly-ness is preserved across media changes (qmp change command).
Similarly, to preserve the snapshot property requires ->open_flags to
be correct.

Signed-off-by: Kevin Shanahan <kmshanah@disenchant.net>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
(cherry picked from commit 80dd1aae3657a902d262f5d20a7a3c655b23705e)

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>