kexec-tools: Make xc_dlhandle static

This patch is a follow-on to commit 894bea93 "kexec-tools: Perform
run-time linking of libxenctrl.so". This patch addresses feedback
from Daniel Kiper.

This patch implements Daniel's suggestion to make the
xc_dlhandle variable static, insert missing 'extern'
qualifier for the new __xc() wrappers, and correct some
style issues.

Signed-off-by: Eric DeVolder <eric.devolder@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
Signed-off-by: Simon Horman <horms@verge.net.au>