ahci_drv default writeable?

[Steven Harp]

Noticed that in 17.08 (and 17.05) these examples failed w. x86/nova
- run/libc_vfs_fs_fat
- run/libc_vfs_fat
apparently because:

 [init -> ahci_drv] read-only session opened at device 0 for 'test-libc_vfs -> 0'

so e.g., mkdir fails with EPERM.

Hacking libports/run/libc_vfs_filesystem_test.inc to add writeable="yes"
to the "ahci_drv" component configuration:
                <config>
                        <policy label_prefix="test-libc_vfs" device="0" writeable="yes"/>
                        <policy label_prefix="vfs" device="0" writeable="yes" />
                </config>
suffices, but maybe this is not appropriate. Was the default policy intended to be
writeable="yes" for this driver?

// Steve Harp