libxl: fix issues in 38cd0664

libxl: fix issues in 38cd0664

A few issues were introduced in 38cd0664 ("libxl/arm: Add the size of
ACPI tables to maxmem"):

1. d_config was not properly initialised and disposed of.
2. using libxl_retrieve_domain_configuration caused thread to
   deadlock itself.

Fix those issues by:

1. properly initialise and dispose of d_config.
2. switch to use libxl__get_domain_configuration.

Note that in theory we can refactor libxl_retrieve_domain_configuration
a bit to get a function without locking, but up until the calculation of
extra memory only relies on static configuration, hence we use the
stored configuration only.

Reported-by: Anthony PERARD <anthony.perard@citrix.com>
Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Tested-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>