contrib/vhost-user-gpu: Fix compiler warning when compiling with -Wshadow

Rename some variables to avoid compiler warnings when compiling
with -Wshadow=local.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-ID: <20231009083726.30301-1-thuth@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>