KVM: arm64: Use scoped_guards in pKVM

The error handling code is often source of bugs because it doesn't get
much love and testing. scoped_guards allow to reduce the error handling
complexity by simplifying greatly locking patterns. This is pretty much
always a good thing, and in particular in security-sensitive code where
bugs are costly.

Convert pKVM to use scoped_guards where possible. No functional changes
intended.

Signed-off-by: Quentin Perret <qperret@google.com>