libxl: avoid compiler warning

libxl: avoid compiler warning

cc1: warnings being treated as errors
libxl/libxl_driver.c: In function 'libxlDomainSetVcpusFlags':
libxl/libxl_driver.c:1570:14: error: cast from function call of type 'double' to non-matching type 'unsigned int' [-Wbad-function-cast]
libxl/libxl_driver.c:1578:15: error: cast from function call of type 'double' to non-matching type 'unsigned int' [-Wbad-function-cast]

This was the only use of floor() and ceil(), and floating-point
is overkill for power-of-two manipulations.

* src/libxl/libxl_driver.c (libxlDomainSetVcpusFlags): Avoid -lm
for trivial computations.