storage: prepare for refactoring
storage: prepare for refactoring

https://bugzilla.redhat.com/show_bug.cgi?id=903248

virStorageFileGetMetadataFromFD is the only caller of
virStorageFileGetMetadataFromBuf; and it doesn't care about the
difference between a return of 0 (total success) or 1
(metadata was inconsistent, but pointer was populated as best
as possible); only about a return of -1 (could not read metadata
or out of memory).  Changing the return type, and normalizing
the variable names used, will make merging the functions easier
in the next commit.

* src/util/virstoragefile.c (virStorageFileGetMetadataFromBuf):
Change return value, and rename some variables.
(virStorageFileGetMetadataFromFD): Rename some variables.
(cherry picked from commit b7df4f92d6a62f4ec7513af11590f77152e0a19b)

mithleshvrts/libvirt-0.10.2,mithleshvrts/libvirt-0.10.2,mithleshvrts/libvirt-0.10.2,mithleshvrts/libvirt-0.10.2,mithleshvrts/libvirt-0.10.2