libxc: Squash xc_e820.h (and delete) into xenctrl.h

libxc: Squash xc_e820.h (and delete) into xenctrl.h

.. as there is no need to keep that internal header file anymore.
We export two functions xc_domain_[set|get]_memory_map which
depend on the 'struct e820entry' defined in 'xc_e820.h'.

We move the contents of the 'xc_e820.h' to the 'xenctrl.h' fixing
compiler errors when applications outside the Xen tree are trying
to compile against the libraries.

Tested-by: Daniel Castro <evil.dani@gmail.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>