MSI-X vectors can be allocated after the initial MSI-X enablement, but this
needs explicit support of the underlying interrupt domains.

Provide a function to query the ability and functions to allocate/free
individual vectors post-enable.

The allocation can either request a specific index in the MSI-X table or
with the index argument MSI_ANY_INDEX it allocates the next free vector.

The return value is a struct msi_map which on success contains both index
and the Linux interrupt number. In case of failure index is negative and
the Linux interrupt number is 0.

The allocation function is for a single MSI-X index at a time as that's
sufficient for the most urgent use case VFIO to get rid of the 'disable
MSI-X, reallocate, enable-MSI-X' cycle which is prone to lost interrupts
and redirections to the legacy and obviously unhandled INTx.

As single index allocation is also sufficient for the use cases Jason
Gunthorpe pointed out: Allocation of a MSI-X or IMS vector for a network
queue. See Link below.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/all/20211126232735.547996838@linutronix.de
Link: https://lore.kernel.org/r/20221124232326.731233614@linutronix.de