pvmfw: Resize stack region to 8 pages

Reduce the number of pages required (which seems to be 4-5 on a normal
boot) to reduce the work when mapping, unmapping, and clearing the
region.

Bug: 270684188
Test: atest MicrodroidHostTests
Change-Id: I626aea440b92c2462f7ca3c761fb66b51a596741
Merged-In: I626aea440b92c2462f7ca3c761fb66b51a596741