x86/hvm: remove multiple open coded 'chunking' loops

x86/hvm: remove multiple open coded 'chunking' loops

...in hvmemul_read/write()

Add hvmemul_phys_mmio_access() and hvmemul_linear_mmio_access() functions
to reduce code duplication.

NOTE: This patch also introduces a change in 'chunking' around a page
      boundary. Previously (for example) an 8 byte access at the last
      byte of a page would get carried out as 8 single-byte accesses.
      It will now be carried out as a single-byte access, followed by
      a 4-byte access, a 2-byte access and then another single-byte
      access.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Cc: Keir Fraser <keir@xen.org>
Cc: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>