util: virdevmapper: Sanitize use of macros for buffer size

util: virdevmapper: Sanitize use of macros for buffer size

There are two distinct uses of an arbitrary buffers size when querying
the device mapper. One is related to loading the /proc/devices file,
while the other is used as buffer for ioctls to the devmapper.

Split up the macros used here so that it's clear that they are not meant
for the same thing.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>