qemu: Add helpers to handle stat data for qemuStorageLimitsRefresh

qemu: Add helpers to handle stat data for qemuStorageLimitsRefresh

Split out the opening of the file and fetch of the stat buffer into a
helper qemuDomainStorageOpenStat. This will handle either opening the
local or remote storage.

Additionally split out the cleanup of that into a separate helper
qemuDomainStorageCloseStat which will either close the file or
call the virStorageFileDeinit function.

Signed-off-by: John Ferlan <jferlan@redhat.com>