arm64: xen: Use existing helper to check interrupt status

The status of interrupts might depend on more than just pstate. Use
interrupts_disabled() instead of raw_irqs_disabled_flags() to take the full
context into account.

Acked-by: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Julien Thierry <julien.thierry@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>