Add showValueList to PyValue for proper String instances
Add showValueList to PyValue for proper String instances

It's the same trick ShowS uses. We add a type class function for
showing a list to PyValue and then just use it in the instance for
`[a]`. This way we have the proper String instance without any
overlapping/incoherent instances.

Signed-off-by: Petr Pudlak <9d19625f8ee825a2c398462ed335cbcf98f79c32@google.com>
Reviewed-by: Jose A. Lopes <0fb9c4f8511df4ec2ae28af879e9d462f6218a2c@google.com>

ganeti-github-testing/ganeti-test-1,andir/ganeti,leshchevds/ganeti,bitemyapp/ganeti,ganeti-github-testing/ganeti-test-1,ganeti/ganeti,bitemyapp/ganeti,mbakke/ganeti,ganeti/ganeti,ganeti/ganeti,yiannist/ganeti,onponomarev/ganeti,andir/ganeti,dimara/ganeti,yiannist/ganeti,grnet/snf-ganeti,onponomarev/ganeti,yiannist/ganeti,grnet/snf-ganeti,dimara/ganeti,mbakke/ganeti,leshchevds/ganeti,apyrgio/ganeti,apyrgio/ganeti,mbakke/ganeti,andir/ganeti,leshchevds/ganeti