storage: Fix build issue with MOUNT and VGCHANGE commands

Turns out there some build platforms that must not define MOUNT
or VGCHANGE in config.h... So moving the commands from the storage
backend specific module into a common storage_util module causes
issues for those platforms.

So instead of assuming they are there, let's just pass the command
string to the storage util API's from the storage backend specific
code (as would have been successful before). Also modify the test
to determine whether the MOUNT and/or VGCHANGE doesn't exist and
just define it to (for example) what Fedora has for the path. Could
have just used "mount" and "vgchange" in the call, but that defeats
the purpose of adding the call to virTestClearCommandPath.

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