Update version to Xen 4.6 RC

Update version to Xen 4.6 RC

* Change README to say `Xen 4.6-rc'
* Change XEN_EXTRAVERSION so that we are `4.6.0-rc'

Note that the RC number (eg, 1 for rc1) is not in the version string,
so that we do not need to update this again when we cut the next RC.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
CC: Jan Beulich <jbeulich@suse.com>
CC: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>