xen: fix physical address availability in dom0

xen: fix physical address availability in dom0

When physical NICs are binded to igb_uio/uio-pci-generic, they cannot
be used in DPDK app in Xen dom0.

Due to (1) a restriction that phys addresses should be availabe is added
by commit cdc242f260e7 ("eal/linux: support running as unprivileged user"),
(2) and previous implementation of the test to check if phys addresses are
available (using a variable on the stack) just works for non-Xen
environment. Actually, for Xen dom0, the physical addresses are always
available if the memory is initialized successfully..

To fix it, we add an precheck to bypass the physical address availability
test.

Fixes: cdc242f260e7 ("eal/linux: support running as unprivileged user")

Signed-off-by: Jianfeng Tan <jianfeng.tan@intel.com>