KVM: selftests: Rename perf_test_util symbols to memstress

Replace the perf_test_ prefix on symbol names with memstress_ to match
the new file name.

"memstress" better describes the functionality proveded by this library,
which is to provide functionality for creating and running a VM that
stresses VM memory by reading and writing to guest memory on all vCPUs
in parallel.

"memstress" also contains the same number of chracters as "perf_test",
making it a drop-in replacement in symbols, e.g. function names, without
impacting line lengths. Also the lack of underscore between "mem" and
"stress" makes it clear "memstress" is a noun.

Signed-off-by: David Matlack <dmatlack@google.com>
Reviewed-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20221012165729.3505266-4-dmatlack@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>