KVM: selftests: Clean up misnomers in xen_shinfo_test

KVM: selftests: Clean up misnomers in xen_shinfo_test

As discussed[*], relabel the poorly named structs to align with the
current KVM nomenclature.

Old names are a leftover from before commit 52491a38b2c2 ("KVM:
Initialize gfn_to_pfn_cache locks in dedicated helper"), which i.a.
introduced kvm_gpc_init() and renamed kvm_gfn_to_pfn_cache_init()/
_destroy() to kvm_gpc_activate()/_deactivate(). Partly in an effort
to avoid implying that the cache really is destroyed/freed.

While at it, get rid of #define GPA_INVALID, which being used as a GFN,
is not only misnamed, but also unnecessarily reinvents a UAPI constant.

No functional change intended.

[*] https://lore.kernel.org/r/Y5yZ6CFkEMBqyJ6v@google.com

Signed-off-by: Michal Luczaj <mhal@rbox.co>
Link: https://lore.kernel.org/r/20230206202430.1898057-1-mhal@rbox.co
Signed-off-by: Sean Christopherson <seanjc@google.com>