vfio: Avoid disabling and enabling vectors repeatedly in VFIO migration

In VFIO migration resume phase and some guest startups, there are
already unmasked vectors in the vector table when calling
vfio_msix_enable(). So in order to avoid inefficiently disabling
and enabling vectors repeatedly, let's allocate all needed vectors
first and then enable these unmasked vectors one by one without
disabling.

Signed-off-by: Shenming Lu <lushenming@huawei.com>
Message-Id: <20210310030233.1133-4-lushenming@huawei.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>