block: Make bdrv_is_writable() public

block: Make bdrv_is_writable() public

This is a useful function for the whole block layer, so make it public.
At the same time, users outside of block.c probably do not need to make
use of the reopen functionality, so rename the current function to
bdrv_is_writable_after_reopen() create a new bdrv_is_writable() function
that just passes NULL to it for the reopen queue.

Cc: [email protected]
Signed-off-by: Max Reitz <[email protected]>
Message-id: 20180606193702[email protected]
Reviewed-by: John Snow <[email protected]>
Reviewed-by: Jeff Cody <[email protected]>
Signed-off-by: Max Reitz <[email protected]>