arm: avoid throwing exceptions in non x86 cases
arm: avoid throwing exceptions in non x86 cases

If we can't detect CPU features, return an empty bitmap instead
of throwing an exception. If we can't read the CPU masks from the
commandline, assume all features are masked.

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@citrix.com>

djs55/xen-api-libs-transitional,thomassa/xen-api-libs-transitional