Introduce a new helper function "virDiskNameParse" which extends
virDiskNameToIndex but handling both disk index and partition index.
Also rework virDiskNameToIndex to be based on virDiskNameParse.
A test is also added for this function testing both valid and
invalid disk names.
Signed-off-by: NJoao Martins <joao.m.martins@oracle.com>