block: Use a GString in bdrv_perm_names()

This is a bit more efficient than having to allocate and free memory
for each new permission.

The default size (30) is enough for "consistent read, write, resize".

Signed-off-by: Alberto Garcia <berto@igalia.com>
Message-id: 20200110171518.22168-1-berto@igalia.com
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>