ANDROID: KVM: arm64: Rename pKVM memcache to stage2_

With the introduction of the pKVM hyp allocator, memcache for VMs and
vCPUs are now only used for carrying pages dedicated to the stage-2.
Hence the renaming to stage2_mc and stage2_teardown_mc.

Bug: 278749606
Bug: 273748186
Change-Id: Ifd1f060682373e95150f087a72fc81ac03f14b8e
Signed-off-by: Vincent Donnefort <vdonnefort@google.com>