ppt: Fix panic when configuring unavailable MSI-X vector

In some cases VM may have different idea about number
of available MSI-X vectors then PPT driver. Return
an error when VM requests setup for more vectors
than expected.

It was observed while using SR-IOV on an Intel E810 Ethernet adapter.
VF driver in a VM sees a correct number of available MSI-X vectors,
which depends on num-queues assigned in iovctl.conf, while
pci_msix_count in the PPT driver always returns 1.

Signed-off-by: Krzysztof Galazka <kgalazka@FreeBSD.org>
Reviewed By: bhyve, jhb
Approved by: erj (mentor), kbowling (mentor)
MFC after: 1 week
Sponsored by: Intel Corporation
Differential Revision: https://reviews.freebsd.org/D48812