vfio: fix interrupts race condition

vfio: fix interrupts race condition

Populating the eventfd in rte_intr_enable in each request to vfio
triggers a reconfiguration of the interrupt handler on the kernel side.
The problem is that rte_intr_enable is often used to re-enable masked
interrupts from drivers interrupt handlers.

This reconfiguration leaves a window during which a device could send
an interrupt and then the kernel logs this (unsolicited from the kernel
point of view) interrupt:
[158764.159833] do_IRQ: 9.34 No irq handler for vector

VFIO api makes it possible to set the fd at setup time.
Make use of this and then we only need to ask for masking/unmasking
legacy interrupts and we have nothing to do for MSI/MSIX.

"rxtx" interrupts are left untouched but are most likely subject to the
same issue.

Reported-at: https://bugzilla.redhat.com/show_bug.cgi?id=1654824
Fixes: 5c782b3928b8 ("vfio: interrupts")
Cc: stable@dpdk.org
Signed-off-by: David Marchand <david.marchand@redhat.com>
Tested-by: Shahed Shaikh <shshaikh@marvell.com>