python: fix -Wsign-compare warnings

python: fix -Wsign-compare warnings

Specifically:
xen/lowlevel/xc/xc.c: In function ‘pyxc_domain_create’:
xen/lowlevel/xc/xc.c:147:24: error: comparison of integer expressions of different signedness: ‘int’ and ‘long unsigned int’ [-Werror=sign-compare]
  147 |         for ( i = 0; i < sizeof(xen_domain_handle_t); i++ )
      |                        ^
xen/lowlevel/xc/xc.c: In function ‘pyxc_domain_sethandle’:
xen/lowlevel/xc/xc.c:312:20: error: comparison of integer expressions of different signedness: ‘int’ and ‘long unsigned int’ [-Werror=sign-compare]
  312 |     for ( i = 0; i < sizeof(xen_domain_handle_t); i++ )
      |                    ^
xen/lowlevel/xc/xc.c: In function ‘pyxc_domain_getinfo’:
xen/lowlevel/xc/xc.c:391:24: error: comparison of integer expressions of different signedness: ‘int’ and ‘long unsigned int’ [-Werror=sign-compare]
  391 |         for ( j = 0; j < sizeof(xen_domain_handle_t); j++ )
      |                        ^
xen/lowlevel/xc/xc.c: In function ‘pyxc_get_device_group’:
xen/lowlevel/xc/xc.c:677:20: error: comparison of integer expressions of different signedness: ‘int’ and ‘uint32_t’ {aka ‘unsigned int’} [-Werror=sign-compare]
  677 |     for ( i = 0; i < num_sdevs; i++ )
      |                    ^
xen/lowlevel/xc/xc.c: In function ‘pyxc_physinfo’:
xen/lowlevel/xc/xc.c:988:20: error: comparison of integer expressions of different signedness: ‘int’ and ‘long unsigned int’ [-Werror=sign-compare]
  988 |     for ( i = 0; i < sizeof(pinfo.hw_cap)/4; i++ )
      |                    ^
xen/lowlevel/xc/xc.c:994:20: error: comparison of integer expressions of different signedness: ‘int’ and ‘long unsigned int’ [-Werror=sign-compare]
  994 |     for ( i = 0; i < ARRAY_SIZE(virtcaps_bits); i++ )
      |                    ^
xen/lowlevel/xc/xc.c:998:24: error: comparison of integer expressions of different signedness: ‘int’ and ‘long unsigned int’ [-Werror=sign-compare]
  998 |         for ( i = 0; i < ARRAY_SIZE(virtcaps_bits); i++ )
      |                        ^
xen/lowlevel/xs/xs.c: In function ‘xspy_ls’:
xen/lowlevel/xs/xs.c:191:23: error: comparison of integer expressions of different signedness: ‘int’ and ‘unsigned int’ [-Werror=sign-compare]
  191 |         for (i = 0; i < xsval_n; i++)
      |                       ^
xen/lowlevel/xs/xs.c: In function ‘xspy_get_permissions’:
xen/lowlevel/xs/xs.c:297:23: error: comparison of integer expressions of different signedness: ‘int’ and ‘unsigned int’ [-Werror=sign-compare]
  297 |         for (i = 0; i < perms_n; i++) {
      |                       ^
cc1: all warnings being treated as errors

Use size_t for loop iterators where it's compared with sizeof() or
similar construct.

Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>