KVM: selftests: Use "gpa" and "gva" for local variable names in pre-fault test

KVM: selftests: Use "gpa" and "gva" for local variable names in pre-fault test

Rename guest_test_{phys,virt}_mem to g{p,v}a in the pre-fault memory test
to shorten line lengths and to use standard terminology.

Opportunsitically use "base_gva" in the guest code instead of "base_gpa"
to match the host side code, which now passes in "gva" (and because
referencing the virtual address avoids having to know that the data is
identity mapped).

No functional change intended.

Cc: Yan Zhao <yan.y.zhao@intel.com>
Link: https://lore.kernel.org/r/20251007224515.374516-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>