util: buffer: Simplify escape buffer allocations

util: buffer: Simplify escape buffer allocations

Replace combinations of xalloc_oversized and VIR_ALLOC_N_QUIET by using
g_malloc0_n which does the checking internally.

This conversion is done with a semantic difference and slightly higher
memory requirements as I've opted to allocate one chunk more than
necessary rather than trying to accomodate the NUL byte separately.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>