mem: Change some default values in the Request class.

mem: Change some default values in the Request class.

These values are more abnormal than the 0s they replace, and so it
would be more obvious when something is accidentally left
uninitialized.

Change-Id: Ie7f14abe9e22f9df1ff238f29d4a783c890f4a20
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/26237
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>