xen: apply XSAs

Apply the following XSAs: 148, 149, 150, 151, 152 and 153. More information
about the content of each XSA can be found at: http://xenbits.xen.org/xsa/

Sponsored by: Citrix Systems R&D
Reviewed by: bapt
Differential revision: https://reviews.freebsd.org/D4049