In order to allow for a virDomainHostdevDef that uses the
virDomainDeviceInfo of a "higher level" device (such as a
virDomainNetDef), this patch changes the virDomainDeviceInfo in the
HostdevDef into a virDomainDeviceInfoPtr. Rather than adding checks
all over the code to check for a null info, we just guarantee that it
is always valid. The new function virDomainHostdevDefAlloc() allocates
a virDomainDeviceInfo and plugs it in, and virDomainHostdevDefFree()
makes sure it is freed.

There were 4 places allocating virDomainHostdevDefs, all of them
parsers of one sort or another, and those have all had their
VIR_ALLOC(hostdev) changed to virDomainHostdevDefAlloc(). Other than
that, and the new functions, all the rest of the changes are just
mechanical removals of "&" or changing "." to "->".