Convert all driver struct intializers to C99 style

Convert all driver struct intializers to C99 style

Change all the driver struct initializers to use the
C99 style, leaving out unused fields. This will make
it possible to add new APIs without changing every
driver. eg change:

    qemudDomainResume, /* domainResume */
    qemudDomainShutdown, /* domainShutdown */
    NULL, /* domainReboot */
    qemudDomainDestroy, /* domainDestroy */

to

    .domainResume = qemudDomainResume,
    .domainShutdown = qemudDomainShutdown,
    .domainDestroy = qemudDomainDestroy,

And get rid of any existing C99 style initializersr which
set NULL, eg change

     .listPools          = vboxStorageListPools,
     .numOfDefinedPools  = NULL,
     .listDefinedPools   = NULL,
     .findPoolSources    = NULL,
     .poolLookupByName   = vboxStoragePoolLookupByName,

to

     .listPools          = vboxStorageListPools,
     .poolLookupByName   = vboxStoragePoolLookupByName,