vfio-pci: Add new function for INTx one-time initialisation

To support INTx enabling for multiple times, we need firstly to extract
one-time initialisation and move the related code into a new function
vfio_pci_init_intx(); if later disable and re-enable the INTx, we can
skip these one-time operations.

This patch move below three main operations for INTx one-time
initialisation from function vfio_pci_enable_intx() into function
vfio_pci_init_intx():

- Reserve 2 FDs for INTx;
- Sanity check with ioctl VFIO_DEVICE_GET_IRQ_INFO;
- Setup pdev->intx_gsi.

Suggested-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
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>