vfio_pci: Test for extended capabilities if config space > 256 bytes

PCI-Express spec says that reading 4 bytes at offset 100h should return
zero if there is no extended capability so VFIO reads this dword to
know if there are extended capabilities.

However it is not always possible to access the extended space so
generic PCI code in pci_cfg_space_size_ext() checks if
pci_read_config_dword() can read beyond 100h and if the check fails,
it sets the config space size to 100h.

VFIO does its own extended capabilities check by reading at offset 100h
which may produce 0xffffffff which VFIO treats as the extended config
space presense and calls vfio_ecap_init() which fails to parse
capabilities (which is expected) but right before the exit, it writes
zero at offset 100h which is beyond the buffer allocated for
vdev->vconfig (which is 256 bytes) which leads to random memory
corruption.

This makes VFIO only check for the extended capabilities if
the discovered config size is more than 256 bytes.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>