vhost: add config flag for postcopy

vhost: add config flag for postcopy

Postcopy live-migration features relies on userfaultfd,
which was only introduced in kernel v4.3.

This patch introduces a new define to allow building vhost
library on kernels not supporting userfaultfd.

With legacy build system, user has to explicitly set
CONFIG_RTE_LIBRTE_VHOST_POSTCOPY to 'y'.

With Meson build system, RTE_LIBRTE_VHOST_POSTCOPY gets
automatically defined if userfaultfd kernel header is
present.

Suggested-by: Ilya Maximets <i.maximets@samsung.com>
Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Acked-by: Ilya Maximets <i.maximets@samsung.com>