xen/common: Introduce _xrealloc function

xen/common: Introduce _xrealloc function

This patch introduces type-unsafe function which besides
re-allocation handles the following corner cases:
1. if requested size is zero, it will behave like xfree
2. if incoming pointer is not valid (NULL or ZERO_BLOCK_PTR),
   it will behave like xmalloc

If both pointer and size are valid the function will re-allocate and
copy only if requested size and alignment don't fit in already
allocated space.

Subsequent patch will add type-safe helper macros.

Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
[julien: comestic changes]
Acked-by: Julien Grall <julien.grall@arm.com>
CC: Andrew Cooper <andrew.cooper3@citrix.com>
CC: George Dunlap <George.Dunlap@eu.citrix.com>
CC: Ian Jackson <ian.jackson@eu.citrix.com>
CC: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
CC: Stefano Stabellini <sstabellini@kernel.org>
CC: Tim Deegan <tim@xen.org>
CC: Wei Liu <wl@xen.org>
CC: Paul Durrant <paul.durrant@citrix.com>