KVM: cleanup and add shortcuts to kvm_range_has_memory_attributes()

Use a guard to simplify early returns, and add two more easy
shortcuts.  If the requested attributes are invalid, the attributes
xarray will never show them as set.  And if testing a single page,
kvm_get_memory_attributes() is more efficient.

Reviewed-by: Michael Roth <michael.roth@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>