Tentatively add new xen-{evtchn,gnt} packages

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
xen-gnt: needs io-page, lwt

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
xen-gnt: depends on libxen-dev

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #8 from djs55/mirage-types-ipaddr

Tentatively add new xen-{evtchn,gnt} packages