block: Add bdrv_reset_options_allowed()

block: Add bdrv_reset_options_allowed()

bdrv_reopen_prepare() receives a BDRVReopenState with (among other
things) a new set of options to be applied to that BlockDriverState.

If an option is missing then it means that we want to reset it to its
default value rather than keep the previous one. This way the state
of the block device after being reopened is comparable to that of a
device added with "blockdev-add" using the same set of options.

Not all options from all drivers can be changed this way, however.
If the user attempts to reset an immutable option to its default value
using this method then we must forbid it.

This new function takes a BlockDriverState and a new set of options
and checks if there's any option that was previously set but is
missing from the new set of options.

If the option is present in both sets we don't need to check that they
have the same value. The loop at the end of bdrv_reopen_prepare()
already takes care of that.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>