Interrupt handling depends on various flags in env->hflags or env->hflags2,
and the exact detail were not exactly replicated between x86_cpu_has_work
and x86_cpu_exec_interrupt.  Create a new function that extracts the
highest-priority non-masked interrupt, and use it in both functions.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>