vfio-pci: Release INTx's unmask eventfd properly

The PCI device INTx uses event fd 'unmask_fd' to signal the deassertion
of the line from guest to host; but this eventfd isn't released properly
when disable INTx.

This patch firstly adds field 'unmask_fd' in struct vfio_pci_device for
storing unmask eventfd and close it when disable INTx.

Signed-off-by: Leo Yan <leo.yan@linaro.org>
Reviewed-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>