[VMX] __vmread() and __vmwrite() now BUG on failure, rather than

[VMX] __vmread() and __vmwrite() now BUG on failure, rather than
returning the error to the caller.

None of our vmcs reads/writes should fail. In fact many callers were
ignoring failures, which can hide bugs (in fact it hid one in
vmx_restore_dr(), which was incorrectly using __vmread() before
the vmcs was loaded).

__vmread_safe() is used by the vmcs dump function, which is the only
function which can legitimately fail a vmcs access.

Signed-off-by: Keir Fraser <keir@xensource.com>