xen/x86: add a function for modifying cr3

xen/x86: add a function for modifying cr3

Instead of having multiple places with more or less identical asm
statements just have one function doing a write to cr3.

As this function should be named write_cr3() rename the current
write_cr3() function to switch_cr3().

Suggested-by: Andrew Copper <andrew.cooper3@citrix.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>