KVM: x86/mmu: Use boolean returns for (S)PTE accessors

Return a 'bool' instead of an 'int' for various PTE accessors that are
boolean in nature, e.g. is_shadow_present_pte().  Returning an int is
goofy and potentially dangerous, e.g. if a flag being checked is moved
into the upper 32 bits of a SPTE, then the compiler may silently squash
the entire check since casting to an int is guaranteed to yield a
return value of '0'.

Opportunistically refactor is_last_spte() so that it naturally returns
a bool value instead of letting it implicitly cast 0/1 to false/true.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210123003003.3137525-1-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>