AHCI: Rework IRQ constants

AHCI: Rework IRQ constants

Create a new enum so that we can name the IRQ bits, which will make debugging
them a little nicer if we can print them out. Not handled in this patch, but
this will make it possible to get a nice debug printf detailing exactly which
status bits are set, as it can be multiple at any given time.

As a consequence of this patch, it is no longer possible to set multiple IRQ
codes at once, but nothing was utilizing this ability anyway.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 20170901001502.29915-8-jsnow@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>