vmw_pvscsi: remove unnecessary internal msi state flag

Internal flag msi_used is uncesessary, msi_uninit() could be called
directly, msi_enabled() is enough to check device msi state.

But for migration compatibility, keep the field in structure.

cc: Paolo Bonzini <pbonzini@redhat.com>
cc: Dmitry Fleytman <dmitry@daynix.com>
cc: Markus Armbruster <armbru@redhat.com>
cc: Marcel Apfelbaum <marcel@redhat.com>
cc: Michael S. Tsirkin <mst@redhat.com>

Signed-off-by: Cao jin <caoj.fnst@cn.fujitsu.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>