mem: Encapsulate mapping gem5 to host address space

mem: Encapsulate mapping gem5 to host address space

Create a function to encapsulate mapping an address in gem5's
address space to the host's address space. The returned value can
be used to access the contents of the given address.

As a side effect, make the local variable hostAddr use snake_case
to comply with gem5's coding style.

Change-Id: I2445d3ab4c7ce5746182b307c26cbafc68aa139c
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22610
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>