block-backend: ignore inserted state in blk_co_nb_sectors

All callers of blk_co_nb_sectors (and blk_nb_sectors) are able to
handle a non-inserted CD-ROM as a zero-length file, they do not need
to raise an error.

Not using blk_co_is_available() aligns the function with
blk_co_get_geometry(), which becomes a simple wrapper for
blk_co_nb_sectors().  It will also make it possible to skip the creation
of a coroutine in the (common) case where bs->bl.has_variable_length
is false.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20230407153303.391121-8-pbonzini@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>