hw/vfio/pci: fix double free in vfio_msi_disable

hw/vfio/pci: fix double free in vfio_msi_disable

The following guest behaviour patter leads to double free in VFIO PCI:

1. Guest enables MSI interrupts
vfio_msi_enable is called, but fails in vfio_enable_vectors.
In our case this was because VFIO GPU device was in D3 state.
Unhappy path in vfio_msi_enable will g_free(vdev->msi_vectors) but not
set this pointer to NULL

2. Guest still sees MSI an enabled after that because emulated config
write is done in vfio_pci_write_config unconditionally before calling
vfio_msi_enable

3. Guest disables MSI interrupts
vfio_msi_disable is called and tries to g_free(vdev->msi_vectors)
in vfio_msi_disable_common => double free

Signed-off-by: Evgeny Yakovlev <wrfsh@yandex-team.ru>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>